diff --git a/gtk/css/gtkcssparser.c b/gtk/css/gtkcssparser.c
index 418368369a..3adddd398d 100644
--- a/gtk/css/gtkcssparser.c
+++ b/gtk/css/gtkcssparser.c
@@ -27,12 +27,13 @@
 #include "gtkcsslocationprivate.h"
 
 typedef struct _GtkCssParserBlock GtkCssParserBlock;
+typedef struct _GtkCssParserTokenStreamData GtkCssParserTokenStreamData;
 
 struct _GtkCssParser
 {
   volatile int ref_count;
 
-  GtkCssTokenizer *tokenizer;
+  GPtrArray *tokenizers;
   GFile *file;
   GFile *directory;
   GtkCssParserErrorFunc error_func;
@@ -42,6 +43,11 @@ struct _GtkCssParser
   GArray *blocks;
   GtkCssLocation location;
   GtkCssToken token;
+
+  GtkCssVariableValue **refs;
+  gsize n_refs;
+  gsize next_ref;
+  gboolean var_fallback;
 };
 
 struct _GtkCssParserBlock
@@ -52,6 +58,12 @@ struct _GtkCssParserBlock
   GtkCssTokenType alternative_token;
 };
 
+static inline GtkCssTokenizer *
+get_tokenizer (GtkCssParser *self)
+{
+  return g_ptr_array_index (self->tokenizers, self->tokenizers->len - 1);
+}
+
 static GtkCssParser *
 gtk_css_parser_new (GtkCssTokenizer       *tokenizer,
                     GFile                 *file,
@@ -64,7 +76,11 @@ gtk_css_parser_new (GtkCssTokenizer       *tokenizer,
   self = g_new0 (GtkCssParser, 1);
 
   self->ref_count = 1;
-  self->tokenizer = gtk_css_tokenizer_ref (tokenizer);
+
+  self->tokenizers = g_ptr_array_new ();
+  g_ptr_array_add (self->tokenizers, gtk_css_tokenizer_ref (tokenizer));
+  g_ptr_array_set_free_func (self->tokenizers, (GDestroyNotify) gtk_css_tokenizer_unref);
+
   if (file)
     {
       self->file = g_object_ref (file);
@@ -117,19 +133,46 @@ gtk_css_parser_new_for_bytes (GBytes                *bytes,
   return result;
 }
 
+GtkCssParser *
+gtk_css_parser_new_for_token_stream (GtkCssVariableValue    *value,
+                                     GFile                  *file,
+                                     GtkCssVariableValue   **refs,
+                                     gsize                   n_refs,
+                                     GtkCssParserErrorFunc   error_func,
+                                     gpointer                user_data,
+                                     GDestroyNotify          user_destroy)
+{
+  GtkCssTokenizer *tokenizer;
+  GtkCssParser *result;
+
+  tokenizer = gtk_css_tokenizer_new_for_range (value->bytes, value->offset,
+                                               value->end_offset - value->offset);
+  result = gtk_css_parser_new (tokenizer, file, error_func, user_data, user_destroy);
+  gtk_css_tokenizer_unref (tokenizer);
+
+  result->refs = refs;
+  result->n_refs = n_refs;
+  result->next_ref = 0;
+
+  return result;
+}
+
 static void
 gtk_css_parser_finalize (GtkCssParser *self)
 {
   if (self->user_destroy)
     self->user_destroy (self->user_data);
 
-  g_clear_pointer (&self->tokenizer, gtk_css_tokenizer_unref);
+  g_clear_pointer (&self->tokenizers, g_ptr_array_unref);
   g_clear_object (&self->file);
   g_clear_object (&self->directory);
   if (self->blocks->len)
     g_critical ("Finalizing CSS parser with %u remaining blocks", self->blocks->len);
   g_array_free (self->blocks, TRUE);
 
+  if (self->refs)
+    g_free (self->refs);
+
   g_free (self);
 }
 
@@ -239,7 +282,7 @@ gtk_css_parser_get_start_location (GtkCssParser *self)
 const GtkCssLocation *
 gtk_css_parser_get_end_location (GtkCssParser *self)
 {
-  return gtk_css_tokenizer_get_location (self->tokenizer);
+  return gtk_css_tokenizer_get_location (get_tokenizer (self));
 }
 
 /**
@@ -273,12 +316,14 @@ static void
 gtk_css_parser_ensure_token (GtkCssParser *self)
 {
   GError *error = NULL;
+  GtkCssTokenizer *tokenizer;
 
   if (!gtk_css_token_is (&self->token, GTK_CSS_TOKEN_EOF))
     return;
 
-  self->location = *gtk_css_tokenizer_get_location (self->tokenizer);
-  if (!gtk_css_tokenizer_read_token (self->tokenizer, &self->token, &error))
+  tokenizer = get_tokenizer (self);
+  self->location = *gtk_css_tokenizer_get_location (tokenizer);
+  if (!gtk_css_tokenizer_read_token (tokenizer, &self->token, &error))
     {
       /* We ignore the error here, because the resulting token will
        * likely already trigger an error in the parsing code and
@@ -286,6 +331,64 @@ gtk_css_parser_ensure_token (GtkCssParser *self)
        */
       g_clear_error (&error);
     }
+
+  if (self->tokenizers->len > 1 && gtk_css_token_is (&self->token, GTK_CSS_TOKEN_EOF))
+    {
+      g_ptr_array_remove_index_fast (self->tokenizers, self->tokenizers->len - 1);
+      gtk_css_parser_ensure_token (self);
+      return;
+    }
+
+  /* Resolve var(--name): skip it and insert the resolved reference instead */
+  if (self->n_refs > 0 && gtk_css_token_is_function (&self->token, "var") && self->var_fallback == 0)
+    {
+      GtkCssVariableValue *ref;
+      GtkCssTokenizer *ref_tokenizer;
+
+      gtk_css_parser_start_block (self);
+
+      if (gtk_css_parser_has_token (self, GTK_CSS_TOKEN_IDENT))
+        {
+          char *var_name = gtk_css_parser_consume_ident (self);
+
+          if (var_name[0] != '-' || var_name[1] != '-')
+            {
+              g_free (var_name);
+
+              self->var_fallback++;
+              gtk_css_parser_skip (self);
+              gtk_css_parser_end_block (self);
+              self->var_fallback--;
+              return;
+            }
+
+          g_free (var_name);
+        }
+      else
+        {
+          self->var_fallback++;
+          gtk_css_parser_skip (self);
+          gtk_css_parser_end_block (self);
+          self->var_fallback--;
+          return;
+        }
+
+      /* If we encounter var() in a fallback when we can already resolve the
+       * actual variable, skip it */
+      self->var_fallback++;
+      gtk_css_parser_skip (self);
+      gtk_css_parser_end_block (self);
+      self->var_fallback--;
+
+      g_assert (self->next_ref < self->n_refs);
+
+      ref = self->refs[self->next_ref++];
+      ref_tokenizer = gtk_css_tokenizer_new_for_range (ref->bytes, ref->offset,
+                                                       ref->end_offset - ref->offset);
+      g_ptr_array_add (self->tokenizers, ref_tokenizer);
+
+      gtk_css_parser_ensure_token (self);
+    }
 }
 
 const GtkCssToken *
@@ -1112,3 +1215,274 @@ gtk_css_parser_consume_any (GtkCssParser            *parser,
 
   return result;
 }
+
+gboolean
+gtk_css_parser_has_references (GtkCssParser *self)
+{
+  GtkCssTokenizer *tokenizer = get_tokenizer (self);
+  gboolean ret = FALSE;
+  int inner_blocks = 0, i;
+
+  gtk_css_tokenizer_save (tokenizer);
+
+  do {
+      const GtkCssToken *token;
+
+      token = gtk_css_parser_get_token (self);
+
+      if (inner_blocks == 0)
+        {
+          if (gtk_css_token_is (token, GTK_CSS_TOKEN_EOF))
+            break;
+
+          if (gtk_css_token_is (token, GTK_CSS_TOKEN_CLOSE_PARENS) ||
+              gtk_css_token_is (token, GTK_CSS_TOKEN_CLOSE_SQUARE))
+            {
+              goto error;
+            }
+        }
+
+      if (gtk_css_token_is_preserved (token, NULL))
+        {
+          if (inner_blocks > 0 && gtk_css_token_is (token, GTK_CSS_TOKEN_EOF))
+            {
+              gtk_css_parser_end_block (self);
+              inner_blocks--;
+            }
+          else
+            {
+              gtk_css_parser_consume_token (self);
+            }
+        }
+      else
+        {
+          gboolean is_var = gtk_css_token_is_function (token, "var");
+
+          inner_blocks++;
+          gtk_css_parser_start_block (self);
+
+          if (!ret && is_var)
+            {
+              token = gtk_css_parser_get_token (self);
+
+              if (gtk_css_token_is (token, GTK_CSS_TOKEN_IDENT))
+                {
+                  const char *var_name = gtk_css_token_get_string (token);
+
+                  if (var_name[0] != '-' || var_name[1] != '-')
+                    goto error;
+
+                  gtk_css_parser_consume_token (self);
+
+                  if (!gtk_css_parser_has_token (self, GTK_CSS_TOKEN_EOF) &&
+                      !gtk_css_parser_has_token (self, GTK_CSS_TOKEN_COMMA))
+                    {
+                      goto error;
+                    }
+
+                  ret = TRUE;
+                }
+            }
+        }
+    }
+  while (!gtk_css_parser_has_token (self, GTK_CSS_TOKEN_SEMICOLON) &&
+         !gtk_css_parser_has_token (self, GTK_CSS_TOKEN_CLOSE_CURLY));
+
+  if (inner_blocks > 0)
+    goto error;
+
+  gtk_css_tokenizer_restore (tokenizer);
+  self->location = *gtk_css_tokenizer_get_location (tokenizer);
+  gtk_css_tokenizer_read_token (tokenizer, &self->token, NULL);
+
+  return ret;
+
+error:
+  for (i = 0; i < inner_blocks; i++)
+    gtk_css_parser_end_block (self);
+
+  gtk_css_tokenizer_restore (tokenizer);
+  self->location = *gtk_css_tokenizer_get_location (tokenizer);
+  gtk_css_tokenizer_read_token (tokenizer, &self->token, NULL);
+
+  return FALSE;
+}
+
+static void
+clear_ref (GtkCssVariableValueReference *ref)
+{
+  g_free (ref->name);
+  if (ref->fallback)
+    gtk_css_variable_value_unref (ref->fallback);
+}
+
+GtkCssVariableValue *
+gtk_css_parser_parse_value_into_token_stream (GtkCssParser *self)
+{
+  GBytes *bytes = gtk_css_tokenizer_get_bytes (get_tokenizer (self));
+  const GtkCssToken *token;
+  gsize offset;
+  gsize length = 0;
+  GArray *refs;
+  GtkCssVariableValueReference *out_refs;
+  gsize n_refs;
+  int inner_blocks = 0, i;
+  gboolean is_initial = FALSE;
+
+  refs = g_array_new (FALSE, TRUE, sizeof (GtkCssVariableValueReference));
+
+  g_array_set_clear_func (refs, (GDestroyNotify) clear_ref);
+
+  for (token = gtk_css_parser_peek_token (self);
+       gtk_css_token_is (token, GTK_CSS_TOKEN_WHITESPACE);
+       token = gtk_css_parser_peek_token (self))
+    {
+      gtk_css_parser_consume_token (self);
+    }
+
+  offset = self->location.bytes;
+
+  do {
+      token = gtk_css_parser_get_token (self);
+
+      if (length == 0 && gtk_css_token_is_ident (token, "initial"))
+        is_initial = TRUE;
+
+      if (gtk_css_token_is (token, GTK_CSS_TOKEN_BAD_STRING) ||
+          gtk_css_token_is (token, GTK_CSS_TOKEN_BAD_URL))
+        {
+          gtk_css_parser_error_syntax (self, "Invalid property value");
+          goto error;
+        }
+
+      if (inner_blocks == 0)
+        {
+          if (gtk_css_token_is (token, GTK_CSS_TOKEN_EOF))
+            break;
+
+          if (gtk_css_token_is (token, GTK_CSS_TOKEN_CLOSE_PARENS) ||
+              gtk_css_token_is (token, GTK_CSS_TOKEN_CLOSE_SQUARE))
+            {
+              gtk_css_parser_error_syntax (self, "Invalid property value");
+              goto error;
+            }
+        }
+
+      if (gtk_css_token_is_preserved (token, NULL))
+        {
+          if (inner_blocks > 0 && gtk_css_token_is (token, GTK_CSS_TOKEN_EOF))
+            {
+              length++;
+              gtk_css_parser_end_block (self);
+
+              inner_blocks--;
+            }
+          else
+            {
+              length++;
+              gtk_css_parser_consume_token (self);
+            }
+        }
+      else
+        {
+          gboolean is_var = gtk_css_token_is_function (token, "var");
+
+          length++;
+          inner_blocks++;
+
+          gtk_css_parser_start_block (self);
+
+          if (is_var)
+            {
+              token = gtk_css_parser_get_token (self);
+
+              if (gtk_css_token_is (token, GTK_CSS_TOKEN_IDENT))
+                {
+                  GtkCssVariableValueReference ref;
+                  char *var_name = g_strdup (gtk_css_token_get_string (token));
+
+                  if (var_name[0] != '-' || var_name[1] != '-')
+                    {
+                      gtk_css_parser_error_value (self, "Invalid variable name: %s", var_name);
+                      g_free (var_name);
+                      goto error;
+                    }
+
+                  length++;
+                  gtk_css_parser_consume_token (self);
+
+                  if (!gtk_css_parser_has_token (self, GTK_CSS_TOKEN_EOF) &&
+                      !gtk_css_parser_has_token (self, GTK_CSS_TOKEN_COMMA))
+                    {
+                      gtk_css_parser_error_syntax (self, "Invalid property value");
+                      g_free (var_name);
+                      goto error;
+                    }
+
+                  ref.name = var_name;
+
+                  if (gtk_css_parser_has_token (self, GTK_CSS_TOKEN_EOF))
+                    {
+                      ref.length = 3;
+                      ref.fallback = NULL;
+                    }
+                  else
+                    {
+                      length++;
+                      gtk_css_parser_consume_token (self);
+
+                      ref.fallback = gtk_css_parser_parse_value_into_token_stream (self);
+
+                      if (ref.fallback == NULL)
+                        {
+                          gtk_css_parser_error_value (self, "Invalid fallback for: %s", var_name);
+                          g_free (var_name);
+                          goto error;
+                        }
+
+                      ref.length = 4 + ref.fallback->length;
+                      length += ref.fallback->length;
+                    }
+
+                  g_array_append_val (refs, ref);
+                }
+            }
+        }
+    }
+  while (!gtk_css_parser_has_token (self, GTK_CSS_TOKEN_SEMICOLON) &&
+         !gtk_css_parser_has_token (self, GTK_CSS_TOKEN_CLOSE_CURLY));
+
+  if (inner_blocks > 0)
+    {
+      gtk_css_parser_error_syntax (self, "Invalid property value");
+      goto error;
+    }
+
+  if (is_initial && length == 1)
+    {
+      g_array_unref (refs);
+
+      return gtk_css_variable_value_new_initial (bytes,
+                                                 offset,
+                                                 self->location.bytes);
+    }
+  else
+    {
+      out_refs = g_array_steal (refs, &n_refs);
+
+      return gtk_css_variable_value_new (bytes,
+                                         offset,
+                                         self->location.bytes,
+                                         length,
+                                         out_refs,
+                                         n_refs);
+    }
+
+error:
+  for (i = 0; i < inner_blocks; i++)
+    gtk_css_parser_end_block (self);
+
+  g_array_unref (refs);
+
+  return NULL;
+}
diff --git a/gtk/css/gtkcssparserprivate.h b/gtk/css/gtkcssparserprivate.h
index c886c5f487..3d837ca283 100644
--- a/gtk/css/gtkcssparserprivate.h
+++ b/gtk/css/gtkcssparserprivate.h
@@ -22,6 +22,7 @@
 
 #include "gtkcssenums.h"
 #include "gtkcsstokenizerprivate.h"
+#include "gtkcssvariablevalueprivate.h"
 
 #include <gio/gio.h>
 
@@ -58,6 +59,13 @@ GtkCssParser *          gtk_css_parser_new_for_bytes            (GBytes
                                                                  GtkCssParserErrorFunc           error_func,
                                                                  gpointer                        user_data,
                                                                  GDestroyNotify                  user_destroy);
+GtkCssParser *          gtk_css_parser_new_for_token_stream     (GtkCssVariableValue            *value,
+                                                                 GFile                          *file,
+                                                                 GtkCssVariableValue           **refs,
+                                                                 gsize                           n_refs,
+                                                                 GtkCssParserErrorFunc           error_func,
+                                                                 gpointer                        user_data,
+                                                                 GDestroyNotify                  user_destroy);
 GtkCssParser *          gtk_css_parser_ref                      (GtkCssParser                   *self);
 void                    gtk_css_parser_unref                    (GtkCssParser                   *self);
 
@@ -150,5 +158,9 @@ gsize                   gtk_css_parser_consume_any              (GtkCssParser
                                                                  gsize                           n_options,
                                                                  gpointer                        user_data);
 
+gboolean                gtk_css_parser_has_references           (GtkCssParser                   *parser);
+
+GtkCssVariableValue *   gtk_css_parser_parse_value_into_token_stream (GtkCssParser              *parser);
+
 G_END_DECLS
 
diff --git a/gtk/css/gtkcsstokenizer.c b/gtk/css/gtkcsstokenizer.c
index 6ac778193a..7a75a9633d 100644
--- a/gtk/css/gtkcsstokenizer.c
+++ b/gtk/css/gtkcsstokenizer.c
@@ -575,6 +575,14 @@ gtk_css_token_init_dimension (GtkCssToken     *token,
 
 GtkCssTokenizer *
 gtk_css_tokenizer_new (GBytes *bytes)
+{
+  return gtk_css_tokenizer_new_for_range (bytes, 0, g_bytes_get_size (bytes));
+}
+
+GtkCssTokenizer *
+gtk_css_tokenizer_new_for_range (GBytes *bytes,
+                                 gsize   offset,
+                                 gsize   length)
 {
   GtkCssTokenizer *tokenizer;
 
@@ -583,8 +591,8 @@ gtk_css_tokenizer_new (GBytes *bytes)
   tokenizer->bytes = g_bytes_ref (bytes);
   tokenizer->name_buffer = g_string_new (NULL);
 
-  tokenizer->data = g_bytes_get_data (bytes, NULL);
-  tokenizer->end = tokenizer->data + g_bytes_get_size (bytes);
+  tokenizer->data = g_bytes_get_region (bytes, 1, offset, length);
+  tokenizer->end = tokenizer->data + length;
 
   gtk_css_location_init (&tokenizer->position);
   tokenizer->saved_states = g_array_new (FALSE, FALSE, sizeof (GtkCssTokenizerSavedState));
@@ -613,6 +621,12 @@ gtk_css_tokenizer_unref (GtkCssTokenizer *tokenizer)
   g_free (tokenizer);
 }
 
+GBytes *
+gtk_css_tokenizer_get_bytes (GtkCssTokenizer *tokenizer)
+{
+  return tokenizer->bytes;
+}
+
 const GtkCssLocation *
 gtk_css_tokenizer_get_location (GtkCssTokenizer *tokenizer)
 {
diff --git a/gtk/css/gtkcsstokenizerprivate.h b/gtk/css/gtkcsstokenizerprivate.h
index 95630b54ad..7b6fe8bfdb 100644
--- a/gtk/css/gtkcsstokenizerprivate.h
+++ b/gtk/css/gtkcsstokenizerprivate.h
@@ -138,10 +138,14 @@ void                    gtk_css_token_print                     (const GtkCssTok
 char *                  gtk_css_token_to_string                 (const GtkCssToken      *token);
 
 GtkCssTokenizer *       gtk_css_tokenizer_new                   (GBytes                 *bytes);
+GtkCssTokenizer *       gtk_css_tokenizer_new_for_range         (GBytes                 *bytes,
+                                                                 gsize                   offset,
+                                                                 gsize                   length);
 
 GtkCssTokenizer *       gtk_css_tokenizer_ref                   (GtkCssTokenizer        *tokenizer);
 void                    gtk_css_tokenizer_unref                 (GtkCssTokenizer        *tokenizer);
 
+GBytes *                gtk_css_tokenizer_get_bytes             (GtkCssTokenizer        *tokenizer);
 const GtkCssLocation *  gtk_css_tokenizer_get_location          (GtkCssTokenizer        *tokenizer) G_GNUC_CONST;
 
 gboolean                gtk_css_tokenizer_read_token            (GtkCssTokenizer        *tokenizer,
diff --git a/gtk/css/gtkcssvariablevalue.c b/gtk/css/gtkcssvariablevalue.c
new file mode 100644
index 0000000000..c829a6a58b
--- /dev/null
+++ b/gtk/css/gtkcssvariablevalue.c
@@ -0,0 +1,141 @@
+/*
+ * Copyright (C) 2023 GNOME Foundation Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authors: Alice Mikhaylenko <alicem@gnome.org>
+ */
+
+#include "gtkcssvariablevalueprivate.h"
+
+GtkCssVariableValue *
+gtk_css_variable_value_new (GBytes                       *bytes,
+                            gsize                         offset,
+                            gsize                         end_offset,
+                            gsize                         length,
+                            GtkCssVariableValueReference *references,
+                            gsize                         n_references)
+{
+  GtkCssVariableValue *self = g_new0 (GtkCssVariableValue, 1);
+
+  self->ref_count = 1;
+
+  self->bytes = g_bytes_ref (bytes);
+  self->offset = offset;
+  self->end_offset = end_offset;
+  self->length = length;
+
+  self->references = references;
+  self->n_references = n_references;
+
+  return self;
+}
+
+GtkCssVariableValue *
+gtk_css_variable_value_new_initial (GBytes *bytes,
+                                    gsize   offset,
+                                    gsize   end_offset)
+{
+  GtkCssVariableValue *self = gtk_css_variable_value_new (bytes, offset, end_offset, 1, NULL, 0);
+
+  self->is_invalid = TRUE;
+
+  return self;
+}
+
+GtkCssVariableValue *
+gtk_css_variable_value_ref (GtkCssVariableValue *self)
+{
+  self->ref_count++;
+
+  return self;
+}
+
+void
+gtk_css_variable_value_unref (GtkCssVariableValue *self)
+{
+  gsize i;
+
+  self->ref_count--;
+  if (self->ref_count > 0)
+    return;
+
+  g_bytes_unref (self->bytes);
+
+  for (i = 0; i < self->n_references; i++)
+    {
+      GtkCssVariableValueReference *ref = &self->references[i];
+
+      g_free (ref->name);
+      if (ref->fallback)
+        gtk_css_variable_value_unref (ref->fallback);
+    }
+
+  g_free (self->references);
+  g_free (self);
+}
+
+void
+gtk_css_variable_value_print (GtkCssVariableValue *self,
+                              GString             *string)
+{
+  gsize len = self->end_offset - self->offset;
+  gconstpointer data = g_bytes_get_region (self->bytes, 1, self->offset, len);
+
+  g_assert (data != NULL);
+
+  g_string_append_len (string, (const char *) data, len);
+}
+
+gboolean
+gtk_css_variable_value_equal (const GtkCssVariableValue *value1,
+                              const GtkCssVariableValue *value2)
+{
+  if (value1 == value2)
+    return TRUE;
+
+  if (value1 == NULL || value2 == NULL)
+    return FALSE;
+
+  if (value1->bytes != value2->bytes)
+    return FALSE;
+
+  if (value1->offset != value2->offset)
+    return FALSE;
+
+  if (value1->end_offset != value2->end_offset)
+    return FALSE;
+
+  return TRUE;
+}
+
+GtkCssVariableValue *
+gtk_css_variable_value_transition  (GtkCssVariableValue *start,
+                                    GtkCssVariableValue *end,
+                                    double               progress)
+{
+  GtkCssVariableValue *ret = progress < 0.5 ? start : end;
+
+  if (ret == NULL)
+    return NULL;
+
+  return gtk_css_variable_value_ref (ret);
+}
+
+void
+gtk_css_variable_value_set_section (GtkCssVariableValue *self,
+                                    GtkCssSection       *section)
+{
+  self->section = gtk_css_section_ref (section);
+}
diff --git a/gtk/css/gtkcssvariablevalueprivate.h b/gtk/css/gtkcssvariablevalueprivate.h
new file mode 100644
index 0000000000..cddc83d9df
--- /dev/null
+++ b/gtk/css/gtkcssvariablevalueprivate.h
@@ -0,0 +1,74 @@
+/*
+ * Copyright (C) 2023 GNOME Foundation Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authors: Alice Mikhaylenko <alicem@gnome.org>
+ */
+
+#pragma once
+
+#include "gtkcss.h"
+#include "gtkcsstokenizerprivate.h"
+
+G_BEGIN_DECLS
+
+typedef struct _GtkCssVariableValueReference GtkCssVariableValueReference;
+typedef struct _GtkCssVariableValue GtkCssVariableValue;
+
+struct _GtkCssVariableValueReference
+{
+  char *name;
+  gsize length;
+  GtkCssVariableValue *fallback;
+};
+
+struct _GtkCssVariableValue
+{
+  int ref_count;
+
+  GBytes *bytes;
+  gsize offset;
+  gsize end_offset;
+  gsize length;
+
+  GtkCssVariableValueReference *references;
+  gsize n_references;
+
+  GtkCssSection *section;
+  gboolean is_invalid;
+};
+
+GtkCssVariableValue *gtk_css_variable_value_new         (GBytes                       *bytes,
+                                                         gsize                         offset,
+                                                         gsize                         end_offset,
+                                                         gsize                         length,
+                                                         GtkCssVariableValueReference *references,
+                                                         gsize                         n_references);
+GtkCssVariableValue *gtk_css_variable_value_new_initial (GBytes                       *bytes,
+                                                         gsize                         offset,
+                                                         gsize                         end_offset);
+GtkCssVariableValue *gtk_css_variable_value_ref         (GtkCssVariableValue          *self);
+void                 gtk_css_variable_value_unref       (GtkCssVariableValue          *self);
+void                 gtk_css_variable_value_print       (GtkCssVariableValue          *self,
+                                                         GString                      *string);
+gboolean             gtk_css_variable_value_equal       (const GtkCssVariableValue    *value1,
+                                                         const GtkCssVariableValue    *value2) G_GNUC_PURE;
+GtkCssVariableValue *gtk_css_variable_value_transition  (GtkCssVariableValue          *start,
+                                                         GtkCssVariableValue          *end,
+                                                         double                        progress);
+void                 gtk_css_variable_value_set_section (GtkCssVariableValue          *self,
+                                                         GtkCssSection                *section);
+
+G_END_DECLS
diff --git a/gtk/css/meson.build b/gtk/css/meson.build
index 9a5f6020b9..8ac659ea7b 100644
--- a/gtk/css/meson.build
+++ b/gtk/css/meson.build
@@ -6,8 +6,9 @@ gtk_css_public_sources = files([
 gtk_css_private_sources = files([
   'gtkcssdataurl.c',
   'gtkcssparser.c',
-  'gtkcsstokenizer.c',
   'gtkcssserializer.c',
+  'gtkcsstokenizer.c',
+  'gtkcssvariablevalue.c',
 ])
 
 gtk_css_public_headers = files([
diff --git a/gtk/gtkcsscustompropertypool.c b/gtk/gtkcsscustompropertypool.c
new file mode 100644
index 0000000000..2131fd287e
--- /dev/null
+++ b/gtk/gtkcsscustompropertypool.c
@@ -0,0 +1,161 @@
+/*
+ * Copyright (C) 2023 GNOME Foundation Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authors: Alice Mikhaylenko <alicem@gnome.org>
+ */
+
+#include "gtkcsscustompropertypoolprivate.h"
+
+struct _GtkCssCustomPropertyPool
+{
+  GObject parent_instance;
+
+  GArray *names;
+  GHashTable *name_mappings;
+};
+
+struct _GtkCssCustomPropertyName
+{
+  int ref_count;
+  char *name;
+};
+
+typedef struct _GtkCssCustomPropertyName GtkCssCustomPropertyName;
+
+static GtkCssCustomPropertyPool *instance = NULL;
+
+G_DEFINE_FINAL_TYPE (GtkCssCustomPropertyPool, gtk_css_custom_property_pool, G_TYPE_OBJECT)
+
+static void
+clear_custom_property_name (GtkCssCustomPropertyName *name)
+{
+  g_clear_pointer (&name->name, g_free);
+}
+
+static void
+gtk_css_custom_property_pool_finalize (GObject *object)
+{
+  GtkCssCustomPropertyPool *self = (GtkCssCustomPropertyPool *)object;
+
+  g_hash_table_unref (self->name_mappings);
+  g_array_unref (self->names);
+
+  G_OBJECT_CLASS (gtk_css_custom_property_pool_parent_class)->finalize (object);
+}
+
+static void
+gtk_css_custom_property_pool_class_init (GtkCssCustomPropertyPoolClass *klass)
+{
+  GObjectClass *object_class = G_OBJECT_CLASS (klass);
+
+  object_class->finalize = gtk_css_custom_property_pool_finalize;
+}
+
+static void
+gtk_css_custom_property_pool_init (GtkCssCustomPropertyPool *self)
+{
+  self->name_mappings = g_hash_table_new (g_str_hash, g_str_equal);
+  self->names = g_array_new (FALSE, FALSE, sizeof (GtkCssCustomPropertyName));
+
+  g_array_set_clear_func (self->names, (GDestroyNotify) clear_custom_property_name);
+}
+
+GtkCssCustomPropertyPool *
+gtk_css_custom_property_pool_get (void)
+{
+  if (instance == NULL)
+    instance = g_object_new (GTK_TYPE_CSS_CUSTOM_PROPERTY_POOL, NULL);
+
+  return instance;
+}
+
+int
+gtk_css_custom_property_pool_add (GtkCssCustomPropertyPool *self,
+                                  const char               *str)
+{
+  GtkCssCustomPropertyName name;
+  int id;
+
+  id = gtk_css_custom_property_pool_lookup (self, str);
+  if (id > 0)
+    return gtk_css_custom_property_pool_ref (self, id);
+
+  name.ref_count = 1;
+  name.name = g_strdup (str);
+
+  // TODO reuse slots after they're gone
+  g_array_append_val (self->names, name);
+
+  id = self->names->len;
+
+  g_hash_table_insert (self->name_mappings, (char *) name.name, GINT_TO_POINTER (id));
+
+  return id;
+}
+
+int
+gtk_css_custom_property_pool_lookup (GtkCssCustomPropertyPool *self,
+                                     const char               *str)
+{
+  gpointer id;
+
+  id = g_hash_table_lookup (self->name_mappings, str);
+
+  return GPOINTER_TO_INT (id);
+}
+
+int
+gtk_css_custom_property_pool_ref (GtkCssCustomPropertyPool *self,
+                                  int                       id)
+{
+  GtkCssCustomPropertyName *name;
+
+  name = &g_array_index (self->names, GtkCssCustomPropertyName, id - 1);
+
+  name->ref_count++;
+
+  return id;
+}
+
+void
+gtk_css_custom_property_pool_unref (GtkCssCustomPropertyPool *self,
+                                    int                       id)
+{
+  GtkCssCustomPropertyName *name;
+
+  name = &g_array_index (self->names, GtkCssCustomPropertyName, id - 1);
+
+  g_assert (name->ref_count > 0);
+
+  name->ref_count--;
+
+  if (name->ref_count == 0)
+    {
+      g_hash_table_remove (self->name_mappings, name->name);
+      clear_custom_property_name (name);
+    }
+}
+
+const char *
+gtk_css_custom_property_pool_get_name (GtkCssCustomPropertyPool *self,
+                                       int                       id)
+{
+  GtkCssCustomPropertyName *name;
+
+  name = &g_array_index (self->names, GtkCssCustomPropertyName, id - 1);
+
+  return name->name;
+}
diff --git a/gtk/gtkcsscustompropertypoolprivate.h b/gtk/gtkcsscustompropertypoolprivate.h
new file mode 100644
index 0000000000..8ae1f6556f
--- /dev/null
+++ b/gtk/gtkcsscustompropertypoolprivate.h
@@ -0,0 +1,43 @@
+/*
+ * Copyright (C) 2023 GNOME Foundation Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authors: Alice Mikhaylenko <alicem@gnome.org>
+ */
+
+#pragma once
+
+#include <glib-object.h>
+
+G_BEGIN_DECLS
+
+#define GTK_TYPE_CSS_CUSTOM_PROPERTY_POOL (gtk_css_custom_property_pool_get_type())
+
+G_DECLARE_FINAL_TYPE (GtkCssCustomPropertyPool, gtk_css_custom_property_pool, GTK, CSS_CUSTOM_PROPERTY_POOL, GObject)
+
+GtkCssCustomPropertyPool *gtk_css_custom_property_pool_get      (void);
+
+int                       gtk_css_custom_property_pool_add      (GtkCssCustomPropertyPool *self,
+                                                                 const char               *str);
+int                       gtk_css_custom_property_pool_lookup   (GtkCssCustomPropertyPool *self,
+                                                                 const char               *str);
+const char *              gtk_css_custom_property_pool_get_name (GtkCssCustomPropertyPool *self,
+                                                                 int                       id);
+int                       gtk_css_custom_property_pool_ref      (GtkCssCustomPropertyPool *self,
+                                                                 int                       id);
+void                      gtk_css_custom_property_pool_unref    (GtkCssCustomPropertyPool *self,
+                                                                 int                       id);
+
+G_END_DECLS
diff --git a/gtk/gtkcsslookup.c b/gtk/gtkcsslookup.c
index 5d7d4112a9..1bf4ce7739 100644
--- a/gtk/gtkcsslookup.c
+++ b/gtk/gtkcsslookup.c
@@ -19,6 +19,7 @@
 
 #include "gtkcsslookupprivate.h"
 
+#include "gtkcsscustompropertypoolprivate.h"
 #include "gtkcssstylepropertyprivate.h"
 #include "gtkcsstypesprivate.h"
 #include "gtkprivatetypebuiltins.h"
@@ -36,6 +37,9 @@ void
 _gtk_css_lookup_destroy (GtkCssLookup *lookup)
 {
   _gtk_bitmask_free (lookup->set_values);
+
+  if (lookup->custom_values)
+    g_hash_table_unref (lookup->custom_values);
 }
 
 gboolean
@@ -74,3 +78,17 @@ _gtk_css_lookup_set (GtkCssLookup  *lookup,
   lookup->values[id].section = section;
   lookup->set_values = _gtk_bitmask_set (lookup->set_values, id, TRUE);
 }
+
+void
+_gtk_css_lookup_set_custom (GtkCssLookup        *lookup,
+                            int                  id,
+                            GtkCssVariableValue *value)
+{
+  gtk_internal_return_if_fail (lookup != NULL);
+
+  if (!lookup->custom_values)
+    lookup->custom_values = g_hash_table_new (g_direct_hash, g_direct_equal);
+
+  if (!g_hash_table_contains (lookup->custom_values, GINT_TO_POINTER (id)))
+    g_hash_table_replace (lookup->custom_values, GINT_TO_POINTER (id), value);
+}
diff --git a/gtk/gtkcsslookupprivate.h b/gtk/gtkcsslookupprivate.h
index 7b0e053ae1..0b8dde87a7 100644
--- a/gtk/gtkcsslookupprivate.h
+++ b/gtk/gtkcsslookupprivate.h
@@ -23,6 +23,8 @@
 #include "gtk/gtkcssstaticstyleprivate.h"
 
 #include "gtk/css/gtkcsssection.h"
+#include "gtk/css/gtkcsstokenizerprivate.h"
+#include "gtk/css/gtkcssvariablevalueprivate.h"
 
 
 G_BEGIN_DECLS
@@ -37,6 +39,7 @@ typedef struct {
 struct _GtkCssLookup {
   GtkBitmask *set_values;
   GtkCssLookupValue  values[GTK_CSS_PROPERTY_N_PROPERTIES];
+  GHashTable *custom_values;
 };
 
 void                    _gtk_css_lookup_init                    (GtkCssLookup               *lookup);
@@ -47,6 +50,9 @@ void                    _gtk_css_lookup_set                     (GtkCssLookup
                                                                  guint                       id,
                                                                  GtkCssSection              *section,
                                                                  GtkCssValue                *value);
+void                    _gtk_css_lookup_set_custom              (GtkCssLookup               *lookup,
+                                                                 int                         id,
+                                                                 GtkCssVariableValue        *value);
 
 static inline const GtkBitmask *
 _gtk_css_lookup_get_set_values (const GtkCssLookup *lookup)
diff --git a/gtk/gtkcssprovider.c b/gtk/gtkcssprovider.c
index b79c38760a..d018524533 100644
--- a/gtk/gtkcssprovider.c
+++ b/gtk/gtkcssprovider.c
@@ -22,10 +22,13 @@
 #include <gtk/css/gtkcss.h>
 #include "gtk/css/gtkcsstokenizerprivate.h"
 #include "gtk/css/gtkcssparserprivate.h"
+#include "gtk/css/gtkcssvariablevalueprivate.h"
 #include "gtkbitmaskprivate.h"
 #include "gtkcssarrayvalueprivate.h"
 #include "gtkcsscolorvalueprivate.h"
+#include "gtkcsscustompropertypoolprivate.h"
 #include "gtkcsskeyframesprivate.h"
+#include "gtkcssreferencevalueprivate.h"
 #include "gtkcssselectorprivate.h"
 #include "gtkcssshorthandpropertyprivate.h"
 #include "gtksettingsprivate.h"
@@ -113,6 +116,7 @@ struct GtkCssRuleset
   PropertyValue *styles;
   guint n_styles;
   guint owns_styles : 1;
+  GHashTable *custom_properties;
 };
 
 struct _GtkCssScanner
@@ -270,6 +274,8 @@ gtk_css_ruleset_clear (GtkCssRuleset *ruleset)
 	    gtk_css_section_unref (ruleset->styles[i].section);
         }
       g_free (ruleset->styles);
+      if (ruleset->custom_properties)
+        g_hash_table_unref (ruleset->custom_properties);
     }
   if (ruleset->selector)
     _gtk_css_selector_free (ruleset->selector);
@@ -285,7 +291,7 @@ gtk_css_ruleset_add (GtkCssRuleset       *ruleset,
 {
   guint i;
 
-  g_return_if_fail (ruleset->owns_styles || ruleset->n_styles == 0);
+  g_return_if_fail (ruleset->owns_styles || (ruleset->n_styles == 0 && ruleset->custom_properties == NULL));
 
   ruleset->owns_styles = TRUE;
 
@@ -315,6 +321,39 @@ gtk_css_ruleset_add (GtkCssRuleset       *ruleset,
     ruleset->styles[i].section = NULL;
 }
 
+static void
+unref_custom_property_name (gpointer pointer)
+{
+  GtkCssCustomPropertyPool *pool = gtk_css_custom_property_pool_get ();
+
+  gtk_css_custom_property_pool_unref (pool, GPOINTER_TO_INT (pointer));
+}
+
+static void
+gtk_css_ruleset_add_custom (GtkCssRuleset       *ruleset,
+                            const char          *name,
+                            GtkCssVariableValue *value)
+{
+  GtkCssCustomPropertyPool *pool;
+  int id;
+
+  g_return_if_fail (ruleset->owns_styles || (ruleset->n_styles == 0 && ruleset->custom_properties == NULL));
+
+  ruleset->owns_styles = TRUE;
+
+  if (ruleset->custom_properties == NULL)
+    {
+      ruleset->custom_properties = g_hash_table_new_full (g_direct_hash, g_direct_equal,
+                                                          unref_custom_property_name,
+                                                          (GDestroyNotify) gtk_css_variable_value_unref);
+    }
+
+  pool = gtk_css_custom_property_pool_get ();
+  id = gtk_css_custom_property_pool_add (pool, name);
+
+  g_hash_table_replace (ruleset->custom_properties, GINT_TO_POINTER (id), value);
+}
+
 static void
 gtk_css_scanner_destroy (GtkCssScanner *scanner)
 {
@@ -490,7 +529,7 @@ gtk_css_style_provider_lookup (GtkStyleProvider             *provider,
         {
           ruleset = gtk_css_selector_matches_get (&tree_rules, i);
 
-          if (ruleset->styles == NULL)
+          if (ruleset->styles == NULL && ruleset->custom_properties == NULL)
             continue;
 
           for (j = 0; j < ruleset->n_styles; j++)
@@ -506,6 +545,18 @@ gtk_css_style_provider_lookup (GtkStyleProvider             *provider,
                                    ruleset->styles[j].section,
                                    ruleset->styles[j].value);
             }
+
+          if (ruleset->custom_properties)
+            {
+              GHashTableIter iter;
+              gpointer id;
+              GtkCssVariableValue *value;
+
+              g_hash_table_iter_init (&iter, ruleset->custom_properties);
+
+              while (g_hash_table_iter_next (&iter, &id, (gpointer) &value))
+                _gtk_css_lookup_set_custom (lookup, GPOINTER_TO_INT (id), value);
+            }
         }
     }
   gtk_css_selector_matches_clear (&tree_rules);
@@ -572,7 +623,7 @@ css_provider_commit (GtkCssProvider  *css_provider,
   GtkCssProviderPrivate *priv = gtk_css_provider_get_instance_private (css_provider);
   guint i;
 
-  if (ruleset->styles == NULL)
+  if (ruleset->styles == NULL && ruleset->custom_properties == NULL)
     {
       for (i = 0; i < gtk_css_selectors_get_size (selectors); i++)
         _gtk_css_selector_free (gtk_css_selectors_get (selectors, i));
@@ -814,6 +865,42 @@ parse_declaration (GtkCssScanner *scanner,
   if (name == NULL)
     goto out;
 
+  /* This is a custom property */
+  if (name[0] == '-' && name[1] == '-')
+    {
+      GtkCssVariableValue *value;
+      GtkCssSection *section;
+
+      if (!gtk_css_parser_try_token (scanner->parser, GTK_CSS_TOKEN_COLON))
+        {
+          gtk_css_parser_error_syntax (scanner->parser, "Expected ':'");
+          goto out;
+        }
+
+      value = gtk_css_parser_parse_value_into_token_stream (scanner->parser);
+      if (value == NULL)
+        goto out;
+
+      if (gtk_keep_css_sections)
+        {
+          section = gtk_css_section_new (gtk_css_parser_get_file (scanner->parser),
+                                         gtk_css_parser_get_block_location (scanner->parser),
+                                         gtk_css_parser_get_end_location (scanner->parser));
+        }
+      else
+        section = NULL;
+
+      if (section != NULL)
+        {
+          gtk_css_variable_value_set_section (value, section);
+          gtk_css_section_unref (section);
+        }
+
+      gtk_css_ruleset_add_custom (ruleset, name, value);
+
+      goto out;
+    }
+
   property = _gtk_style_property_lookup (name);
 
   if (property)
@@ -827,15 +914,60 @@ parse_declaration (GtkCssScanner *scanner,
           goto out;
         }
 
-      value = _gtk_style_property_parse_value (property, scanner->parser);
-
-      if (value == NULL)
-        goto out;
-
-      if (!gtk_css_parser_has_token (scanner->parser, GTK_CSS_TOKEN_EOF))
+      if (gtk_css_parser_has_references (scanner->parser))
         {
-          gtk_css_parser_error_syntax (scanner->parser, "Junk at end of value for %s", property->name);
-          goto out;
+          GtkCssVariableValue *var_value;
+
+          var_value = gtk_css_parser_parse_value_into_token_stream (scanner->parser);
+          if (var_value == NULL)
+            goto out;
+
+          if (GTK_IS_CSS_SHORTHAND_PROPERTY (property))
+            {
+              GtkCssShorthandProperty *shorthand = GTK_CSS_SHORTHAND_PROPERTY (property);
+              guint i, n;
+              GtkCssValue **values;
+
+              n = _gtk_css_shorthand_property_get_n_subproperties (shorthand);
+
+              values = g_new (GtkCssValue *, n);
+
+              for (i = 0; i < n; i++)
+                {
+                  GtkCssValue *child =
+                    _gtk_css_reference_value_new (property,
+                                                  var_value,
+                                                  gtk_css_parser_get_file (scanner->parser));
+                  _gtk_css_reference_value_set_subproperty (child, i);
+
+                  values[i] = _gtk_css_array_value_get_nth (child, i);
+                }
+
+              value = _gtk_css_array_value_new_from_array (values, n);
+              g_free (values);
+            }
+          else
+            {
+              value = _gtk_css_reference_value_new (property,
+                                                    var_value,
+                                                    gtk_css_parser_get_file (scanner->parser));
+            }
+
+          gtk_css_variable_value_unref (var_value);
+        }
+      else
+        {
+          value = _gtk_style_property_parse_value (property, scanner->parser);
+
+          if (value == NULL)
+            goto out;
+
+          if (!gtk_css_parser_has_token (scanner->parser, GTK_CSS_TOKEN_EOF))
+            {
+              gtk_css_parser_error_syntax (scanner->parser, "Junk at end of value for %s", property->name);
+              gtk_css_value_unref (value);
+              goto out;
+            }
         }
 
       if (gtk_keep_css_sections)
@@ -1473,6 +1605,20 @@ compare_properties (gconstpointer a, gconstpointer b, gpointer style)
                  _gtk_style_property_get_name (GTK_STYLE_PROPERTY (styles[*ub].property)));
 }
 
+static int
+compare_custom_properties (gconstpointer a, gconstpointer b, gpointer user_data)
+{
+  GtkCssCustomPropertyPool *pool = user_data;
+  int id1 = GPOINTER_TO_INT (*((gconstpointer *) a));
+  int id2 = GPOINTER_TO_INT (*((gconstpointer *) b));
+  const char *name1, *name2;
+
+  name1 = gtk_css_custom_property_pool_get_name (pool, id1);
+  name2 = gtk_css_custom_property_pool_get_name (pool, id2);
+
+  return strcmp (name1, name2);
+}
+
 static void
 gtk_css_ruleset_print (const GtkCssRuleset *ruleset,
                        GString             *str)
@@ -1503,6 +1649,31 @@ gtk_css_ruleset_print (const GtkCssRuleset *ruleset,
           g_string_append (str, ";\n");
         }
 
+        if (ruleset->custom_properties)
+          {
+            GtkCssCustomPropertyPool *pool = gtk_css_custom_property_pool_get ();
+            GPtrArray *keys;
+
+            keys = g_hash_table_get_keys_as_ptr_array (ruleset->custom_properties);
+            g_ptr_array_sort_with_data (keys, compare_custom_properties, pool);
+
+            for (i = 0; i < keys->len; i++)
+              {
+                int id = GPOINTER_TO_INT (g_ptr_array_index (keys, i));
+                const char *name = gtk_css_custom_property_pool_get_name (pool, id);
+                GtkCssVariableValue *value = g_hash_table_lookup (ruleset->custom_properties,
+                                                                  GINT_TO_POINTER (id));
+
+                g_string_append (str, "  ");
+                g_string_append (str, name);
+                g_string_append (str, ": ");
+                gtk_css_variable_value_print (value, str);
+                g_string_append (str, ";\n");
+              }
+
+            g_ptr_array_unref (keys);
+          }
+
       g_free (sorted);
     }
 
diff --git a/gtk/gtkcssreferencevalue.c b/gtk/gtkcssreferencevalue.c
new file mode 100644
index 0000000000..ba234639f2
--- /dev/null
+++ b/gtk/gtkcssreferencevalue.c
@@ -0,0 +1,268 @@
+/*
+ * Copyright (C) 2023 GNOME Foundation Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authors: Alice Mikhaylenko <alicem@gnome.org>
+ */
+
+#include "config.h"
+
+#include "gtkcssreferencevalueprivate.h"
+
+#include "gtkcssarrayvalueprivate.h"
+#include "gtkcsscustompropertypoolprivate.h"
+#include "gtkcssshorthandpropertyprivate.h"
+#include "gtkcssstyleprivate.h"
+#include "gtkcssunsetvalueprivate.h"
+#include "gtkcssvalueprivate.h"
+#include "gtkstyleproviderprivate.h"
+
+#define MAX_TOKEN_LENGTH 65536
+
+struct _GtkCssValue {
+  GTK_CSS_VALUE_BASE
+
+  GtkStyleProperty *property;
+  GtkCssVariableValue *value;
+  GFile *file;
+  guint subproperty;
+};
+
+static void
+gtk_css_value_reference_free (GtkCssValue *value)
+{
+  gtk_css_variable_value_unref (value->value);
+  if (value->file)
+    g_object_unref (value->file);
+}
+
+static gboolean
+resolve_references_do (GtkCssVariableValue *value,
+                       GtkCssVariableSet   *style_variables,
+                       gboolean             root,
+                       GPtrArray           *refs,
+                       gsize               *out_length,
+                       gsize               *out_n_refs)
+{
+  GtkCssCustomPropertyPool *pool = gtk_css_custom_property_pool_get ();
+  gsize i;
+  gsize length = value->length;
+  gsize n_refs = 0;
+
+  if (value->is_invalid)
+    goto error;
+
+  if (!root)
+    {
+      n_refs += 1;
+      g_ptr_array_add (refs, value);
+    }
+
+  for (i = 0; i < value->n_references; i++)
+    {
+      GtkCssVariableValueReference *ref = &value->references[i];
+      int id = gtk_css_custom_property_pool_lookup (pool, ref->name);
+      GtkCssVariableValue *var_value = NULL;
+      gsize var_length, var_refs;
+      GtkCssVariableSet *source = style_variables;
+
+      if (!var_value && style_variables)
+        var_value = gtk_css_variable_set_lookup (style_variables, id, &source);
+
+      if (!var_value || !resolve_references_do (var_value, source,
+                                                FALSE, refs, &var_length, &var_refs))
+        {
+          var_value = ref->fallback;
+
+          if (!var_value || !resolve_references_do (var_value, style_variables,
+                                                    FALSE, refs, &var_length, &var_refs))
+            goto error;
+        }
+
+      length += var_length - ref->length;
+      n_refs += var_refs;
+
+      if (length > MAX_TOKEN_LENGTH)
+        goto error;
+    }
+
+  if (out_length)
+    *out_length = length;
+
+  if (out_n_refs)
+    *out_n_refs = n_refs;
+
+  return TRUE;
+
+error:
+  /* Remove the references we added as if nothing happened */
+  g_ptr_array_remove_range (refs, refs->len - n_refs, n_refs);
+
+  if (out_length)
+    *out_length = 0;
+
+  if (out_n_refs)
+    *out_n_refs = 0;
+
+  return FALSE;
+}
+
+static GtkCssVariableValue **
+resolve_references (GtkCssVariableValue *input,
+                    GtkCssStyle         *style,
+                    gsize               *n_refs)
+{
+  GPtrArray *refs = g_ptr_array_new ();
+
+  if (!resolve_references_do (input, style->variables, TRUE, refs, NULL, NULL))
+    return NULL;
+
+  return (GtkCssVariableValue **) g_ptr_array_steal (refs, n_refs);
+}
+
+static void
+parser_error (GtkCssParser         *parser,
+              const GtkCssLocation *start,
+              const GtkCssLocation *end,
+              const GError         *error,
+              gpointer              user_data)
+{
+  GtkStyleProvider *provider = user_data;
+  GtkCssSection *section;
+
+  section = gtk_css_section_new (gtk_css_parser_get_file (parser),
+                                 start,
+                                 end);
+
+  gtk_style_provider_emit_error (provider, section, (GError *) error);
+
+  gtk_css_section_unref (section);
+}
+
+static GtkCssValue *
+gtk_css_value_reference_compute (GtkCssValue      *value,
+                                 guint             property_id,
+                                 GtkStyleProvider *provider,
+                                 GtkCssStyle      *style,
+                                 GtkCssStyle      *parent_style)
+{
+  GtkCssValue *result = NULL, *computed;
+  GtkCssVariableValue **refs;
+  gsize n_refs = 0;
+
+  refs = resolve_references (value->value, style, &n_refs);
+
+  if (refs != NULL)
+    {
+      GtkCssParser *value_parser =
+        gtk_css_parser_new_for_token_stream (value->value,
+                                             value->file,
+                                             refs,
+                                             n_refs,
+                                             parser_error, provider, NULL);
+
+      result = _gtk_style_property_parse_value (value->property, value_parser);
+      gtk_css_parser_unref (value_parser);
+    }
+
+  if (result == NULL)
+    result = _gtk_css_unset_value_new ();
+
+  if (GTK_IS_CSS_SHORTHAND_PROPERTY (value->property))
+    {
+      GtkCssValue *sub = gtk_css_value_ref (_gtk_css_array_value_get_nth (result, value->subproperty));
+      gtk_css_value_unref (result);
+      result = sub;
+    }
+
+  computed = _gtk_css_value_compute (result,
+                                     property_id,
+                                     provider,
+                                     style,
+                                     parent_style);
+
+  gtk_css_value_unref (result);
+
+  return computed;
+}
+
+static gboolean
+gtk_css_value_reference_equal (const GtkCssValue *value1,
+                               const GtkCssValue *value2)
+{
+  return FALSE;
+}
+
+static GtkCssValue *
+gtk_css_value_reference_transition (GtkCssValue *start,
+                                    GtkCssValue *end,
+                                    guint        property_id,
+                                    double       progress)
+{
+  return NULL;
+}
+
+static gboolean
+gtk_css_value_reference_contains_variables (const GtkCssValue *value)
+{
+  return TRUE;
+}
+
+static void
+gtk_css_value_reference_print (const GtkCssValue *value,
+                               GString           *string)
+{
+  gtk_css_variable_value_print (value->value, string);
+}
+
+static const GtkCssValueClass GTK_CSS_VALUE_REFERENCE = {
+  "GtkCssReferenceValue",
+  gtk_css_value_reference_free,
+  gtk_css_value_reference_compute,
+  gtk_css_value_reference_equal,
+  gtk_css_value_reference_transition,
+  NULL,
+  NULL,
+  gtk_css_value_reference_contains_variables,
+  gtk_css_value_reference_print
+};
+
+GtkCssValue *
+_gtk_css_reference_value_new (GtkStyleProperty    *property,
+                              GtkCssVariableValue *value,
+                              GFile               *file)
+{
+  GtkCssValue *result;
+
+  result = _gtk_css_value_new (GtkCssValue, &GTK_CSS_VALUE_REFERENCE);
+  result->property = property;
+  result->value = gtk_css_variable_value_ref (value);
+
+  if (file)
+    result->file = g_object_ref (file);
+  else
+    result->file = NULL;
+
+  return result;
+}
+
+void
+_gtk_css_reference_value_set_subproperty (GtkCssValue *value,
+                                          guint        property)
+{
+  g_assert (GTK_IS_CSS_SHORTHAND_PROPERTY (value->property));
+
+  value->subproperty = property;
+}
diff --git a/gtk/gtkcssreferencevalueprivate.h b/gtk/gtkcssreferencevalueprivate.h
new file mode 100644
index 0000000000..13feca983c
--- /dev/null
+++ b/gtk/gtkcssreferencevalueprivate.h
@@ -0,0 +1,35 @@
+/*
+ * Copyright (C) 2023 GNOME Foundation Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authors: Alice Mikhaylenko <alicem@gnome.org>
+ */
+
+#pragma once
+
+#include <gtk/css/gtkcss.h>
+#include "gtkcssvalueprivate.h"
+#include "gtkstylepropertyprivate.h"
+#include "css/gtkcssvariablevalueprivate.h"
+
+G_BEGIN_DECLS
+
+GtkCssValue *_gtk_css_reference_value_new             (GtkStyleProperty    *property,
+                                                       GtkCssVariableValue *value,
+                                                       GFile               *file);
+void         _gtk_css_reference_value_set_subproperty (GtkCssValue         *value,
+                                                       guint                property);
+
+G_END_DECLS
diff --git a/gtk/gtkcssstaticstyle.c b/gtk/gtkcssstaticstyle.c
index 71a10bc552..098deaaaf2 100644
--- a/gtk/gtkcssstaticstyle.c
+++ b/gtk/gtkcssstaticstyle.c
@@ -912,6 +912,35 @@ gtk_css_lookup_resolve (GtkCssLookup      *lookup,
   gtk_internal_return_if_fail (GTK_IS_CSS_STATIC_STYLE (style));
   gtk_internal_return_if_fail (parent_style == NULL || GTK_IS_CSS_STYLE (parent_style));
 
+  if (lookup->custom_values)
+    {
+      GHashTableIter iter;
+      gpointer id;
+      GtkCssVariableValue *value;
+
+      style->variables = gtk_css_variable_set_new ();
+
+      g_hash_table_iter_init (&iter, lookup->custom_values);
+
+      while (g_hash_table_iter_next (&iter, &id, (gpointer) &value))
+        gtk_css_variable_set_add (style->variables, GPOINTER_TO_INT (id), value);
+
+      gtk_css_variable_set_resolve_cycles (style->variables);
+
+      if (parent_style)
+        {
+          GtkCssStyle *parent_sstyle = GTK_CSS_STYLE (parent_style);
+          gtk_css_variable_set_set_parent (style->variables,
+                                           parent_sstyle->variables);
+        }
+    }
+  else if (parent_style)
+    {
+      GtkCssStyle *parent_sstyle = GTK_CSS_STYLE (parent_style);
+      if (parent_sstyle->variables)
+        style->variables = gtk_css_variable_set_ref (parent_sstyle->variables);
+    }
+
   if (_gtk_bitmask_is_empty (_gtk_css_lookup_get_set_values (lookup)))
     {
       style->background = (GtkCssBackgroundValues *)gtk_css_values_ref (gtk_css_background_initial_values);
@@ -1107,3 +1136,15 @@ gtk_css_static_style_get_change (GtkCssStaticStyle *style)
 
   return style->change;
 }
+
+void
+gtk_css_custom_values_compute_changes_and_affects (GtkCssStyle    *style1,
+                                                   GtkCssStyle    *style2,
+                                                   GtkBitmask    **changes,
+                                                   GtkCssAffects  *affects)
+{
+  if (gtk_css_variable_set_equal (style1->variables, style2->variables))
+    return;
+
+  *changes = _gtk_bitmask_set (*changes, GTK_CSS_PROPERTY_CUSTOM, TRUE);
+}
diff --git a/gtk/gtkcssstyle.c b/gtk/gtkcssstyle.c
index 0822e80a46..0f9ee23c54 100644
--- a/gtk/gtkcssstyle.c
+++ b/gtk/gtkcssstyle.c
@@ -24,6 +24,7 @@
 
 #include "gtkcssanimationprivate.h"
 #include "gtkcssarrayvalueprivate.h"
+#include "gtkcsscustompropertypoolprivate.h"
 #include "gtkcssenumvalueprivate.h"
 #include "gtkcssinheritvalueprivate.h"
 #include "gtkcssinitialvalueprivate.h"
@@ -73,6 +74,9 @@ gtk_css_style_finalize (GObject *object)
   gtk_css_values_unref ((GtkCssValues *)style->size);
   gtk_css_values_unref ((GtkCssValues *)style->other);
 
+  if (style->variables)
+    gtk_css_variable_set_unref (style->variables);
+
   G_OBJECT_CLASS (gtk_css_style_parent_class)->finalize (object);
 }
 
@@ -368,6 +372,34 @@ gtk_css_style_print (GtkCssStyle *style,
       retval = TRUE;
     }
 
+    if (style->variables)
+      {
+        GtkCssCustomPropertyPool *pool = gtk_css_custom_property_pool_get ();
+        GArray *ids = gtk_css_variable_set_list_ids (style->variables);
+
+        for (i = 0; i < ids->len; i++)
+          {
+            int id = g_array_index (ids, int, i);
+            const char *name = gtk_css_custom_property_pool_get_name (pool, id);
+            GtkCssVariableValue *value = gtk_css_variable_set_lookup (style->variables, id, NULL);
+
+            g_string_append_printf (string, "%*s%s: ", indent, "", name);
+            gtk_css_variable_value_print (value, string);
+            g_string_append_c (string, ';');
+
+            if (value->section)
+              {
+                g_string_append (string, " /* ");
+                gtk_css_section_print (value->section, string);
+                g_string_append (string, " */");
+              }
+
+            g_string_append_c (string, '\n');
+          }
+
+        retval = TRUE;
+      }
+
   return retval;
 }
 
@@ -837,3 +869,13 @@ gtk_css_values_new (GtkCssValuesType type)
 
   return values;
 }
+
+GtkCssVariableValue *
+gtk_css_style_get_custom_property (GtkCssStyle *style,
+                                   int          id)
+{
+  if (style->variables)
+    return gtk_css_variable_set_lookup (style->variables, id, NULL);
+
+  return NULL;
+}
diff --git a/gtk/gtkcssstylechange.c b/gtk/gtkcssstylechange.c
index 3792b0fe51..1405d35d3c 100644
--- a/gtk/gtkcssstylechange.c
+++ b/gtk/gtkcssstylechange.c
@@ -102,6 +102,12 @@ compute_change (GtkCssStyleChange *change)
                                                       change->new_style,
                                                       &change->changes,
                                                       &change->affects);
+
+  if (change->old_style->variables != change->new_style->variables)
+    gtk_css_custom_values_compute_changes_and_affects (change->old_style,
+                                                       change->new_style,
+                                                       &change->changes,
+                                                       &change->affects);
 }
 
 void
diff --git a/gtk/gtkcssstyleprivate.h b/gtk/gtkcssstyleprivate.h
index 82531a6ffe..e3e3364b2d 100644
--- a/gtk/gtkcssstyleprivate.h
+++ b/gtk/gtkcssstyleprivate.h
@@ -24,6 +24,8 @@
 
 #include "gtk/gtkbitmaskprivate.h"
 #include "gtk/gtkcssvalueprivate.h"
+#include "gtk/gtkcssvariablesetprivate.h"
+#include "gtk/css/gtkcssvariablevalueprivate.h"
 
 G_BEGIN_DECLS
 
@@ -232,6 +234,10 @@ struct _GtkCssStyle
   GtkCssTransitionValues  *transition;
   GtkCssSizeValues        *size;
   GtkCssOtherValues       *other;
+  GtkCssVariableSet       *variables;
+
+  GtkCssValue             *variable_values;
+  int                      n_variable_values;
 };
 
 struct _GtkCssStyleClass
@@ -268,6 +274,9 @@ char *                  gtk_css_style_compute_font_features     (GtkCssStyle
 PangoAttrList *         gtk_css_style_get_pango_attributes      (GtkCssStyle            *style);
 PangoFontDescription *  gtk_css_style_get_pango_font            (GtkCssStyle            *style);
 
+GtkCssVariableValue *   gtk_css_style_get_custom_property       (GtkCssStyle            *style,
+                                                                int                      id);
+
 GtkCssValues *gtk_css_values_new   (GtkCssValuesType  type);
 GtkCssValues *gtk_css_values_ref   (GtkCssValues     *values);
 void          gtk_css_values_unref (GtkCssValues     *values);
@@ -317,6 +326,10 @@ void gtk_css_other_values_compute_changes_and_affects (GtkCssStyle *style1,
                                                       GtkCssStyle *style2,
                                                       GtkBitmask    **changes,
                                                       GtkCssAffects *affects);
+void gtk_css_custom_values_compute_changes_and_affects (GtkCssStyle *style1,
+                                                      GtkCssStyle *style2,
+                                                      GtkBitmask    **changes,
+                                                      GtkCssAffects *affects);
 
 G_END_DECLS
 
diff --git a/gtk/gtkcsstypesprivate.h b/gtk/gtkcsstypesprivate.h
index 96eeb6b3c8..7141248188 100644
--- a/gtk/gtkcsstypesprivate.h
+++ b/gtk/gtkcsstypesprivate.h
@@ -277,7 +277,8 @@ enum { /*< skip >*/
   GTK_CSS_PROPERTY_FONT_VARIATION_SETTINGS,
   GTK_CSS_PROPERTY_LINE_HEIGHT,
   /* add more */
-  GTK_CSS_PROPERTY_N_PROPERTIES
+  GTK_CSS_PROPERTY_N_PROPERTIES,
+  GTK_CSS_PROPERTY_CUSTOM,
 };
 
 typedef enum /*< skip >*/ {
diff --git a/gtk/gtkcssvariableset.c b/gtk/gtkcssvariableset.c
new file mode 100644
index 0000000000..96a8be3426
--- /dev/null
+++ b/gtk/gtkcssvariableset.c
@@ -0,0 +1,337 @@
+/*
+ * Copyright (C) 2023 GNOME Foundation Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authors: Alice Mikhaylenko <alicem@gnome.org>
+ */
+
+#include "gtkcssvariablesetprivate.h"
+
+#include "gtkcsscustompropertypoolprivate.h"
+
+static void
+unref_custom_property_name (gpointer pointer)
+{
+  GtkCssCustomPropertyPool *pool = gtk_css_custom_property_pool_get ();
+
+  gtk_css_custom_property_pool_unref (pool, GPOINTER_TO_INT (pointer));
+}
+
+static void
+maybe_unref_variable (gpointer pointer)
+{
+  if (pointer != NULL)
+    gtk_css_variable_value_unref (pointer);
+}
+
+GtkCssVariableSet *
+gtk_css_variable_set_new (void)
+{
+  GtkCssVariableSet *self = g_new0 (GtkCssVariableSet, 1);
+
+  self->ref_count = 1;
+  self->variables = g_hash_table_new_full (g_direct_hash,
+                                           g_direct_equal,
+                                           unref_custom_property_name,
+                                           maybe_unref_variable);
+
+  return self;
+}
+
+GtkCssVariableSet *
+gtk_css_variable_set_ref (GtkCssVariableSet *self)
+{
+  self->ref_count++;
+
+  return self;
+}
+
+void
+gtk_css_variable_set_unref (GtkCssVariableSet *self)
+{
+  self->ref_count--;
+  if (self->ref_count > 0)
+    return;
+
+  g_hash_table_unref (self->variables);
+  g_clear_pointer (&self->parent, gtk_css_variable_set_unref);
+  g_free (self);
+}
+
+GtkCssVariableSet *
+gtk_css_variable_set_copy (GtkCssVariableSet *self)
+{
+  GtkCssVariableSet *ret = gtk_css_variable_set_new ();
+  GHashTableIter iter;
+  gpointer id;
+  GtkCssVariableValue *value;
+
+  g_hash_table_iter_init (&iter, self->variables);
+
+  while (g_hash_table_iter_next (&iter, &id, (gpointer) &value))
+    gtk_css_variable_set_add (ret, GPOINTER_TO_INT (id), value);
+
+  gtk_css_variable_set_set_parent (ret, self->parent);
+
+  return ret;
+}
+
+void
+gtk_css_variable_set_set_parent (GtkCssVariableSet *self,
+                                 GtkCssVariableSet *parent)
+{
+  if (self->parent)
+    gtk_css_variable_set_unref (self->parent);
+
+  self->parent = parent;
+
+  if (self->parent)
+    gtk_css_variable_set_ref (self->parent);
+}
+
+void
+gtk_css_variable_set_add (GtkCssVariableSet   *self,
+                          int                  id,
+                          GtkCssVariableValue *value)
+{
+  GtkCssCustomPropertyPool *pool = gtk_css_custom_property_pool_get ();
+
+  g_hash_table_insert (self->variables,
+                       GINT_TO_POINTER (gtk_css_custom_property_pool_ref (pool, id)),
+                       value ? gtk_css_variable_value_ref (value) : NULL);
+}
+
+static gboolean check_variable (GtkCssVariableSet   *self,
+                                GHashTable          *unvisited_variables,
+                                GArray              *stack,
+                                int                  id,
+                                GtkCssVariableValue *value);
+
+static gboolean
+check_references (GtkCssVariableSet   *self,
+                  GHashTable          *unvisited_variables,
+                  GArray              *stack,
+                  GtkCssVariableValue *value)
+{
+  GtkCssCustomPropertyPool *pool = gtk_css_custom_property_pool_get ();
+  int i;
+
+  for (i = 0; i < value->n_references; i++)
+    {
+      GtkCssVariableValueReference *ref = &value->references[i];
+      int ref_id = gtk_css_custom_property_pool_lookup (pool, ref->name);
+
+      if (g_hash_table_contains (unvisited_variables, GINT_TO_POINTER (ref_id)))
+        {
+          GtkCssVariableValue *ref_value;
+
+          ref_value = g_hash_table_lookup (self->variables, GINT_TO_POINTER (ref_id));
+
+          if (check_variable (self, unvisited_variables, stack, ref_id, ref_value))
+            return TRUE;
+        }
+
+      if (ref->fallback)
+        {
+          if (check_references (self, unvisited_variables, stack, ref->fallback))
+            return TRUE;
+        }
+    }
+
+    return FALSE;
+}
+
+static gboolean
+check_variable (GtkCssVariableSet   *self,
+                GHashTable          *unvisited_variables,
+                GArray              *stack,
+                int                  id,
+                GtkCssVariableValue *value)
+{
+  int i;
+
+  g_array_append_val (stack, id);
+
+  for (i = stack->len - 2; i >= 0; i--)
+    {
+      /* Found a cycle, stop checking */
+      if (id == g_array_index (stack, int, i))
+        return TRUE;
+    }
+
+  if (check_references (self, unvisited_variables, stack, value))
+    return TRUE;
+
+  g_array_remove_index_fast (stack, stack->len - 1);
+  g_hash_table_remove (unvisited_variables, GINT_TO_POINTER (id));
+
+  return FALSE;
+}
+
+void
+gtk_css_variable_set_resolve_cycles (GtkCssVariableSet *self)
+{
+  GHashTable *variables = g_hash_table_new (g_direct_hash, g_direct_equal);
+  GHashTable *unvisited_variables = g_hash_table_new (g_direct_hash, g_direct_equal);
+  GArray *stack;
+  GHashTableIter iter;
+  gpointer id;
+  GtkCssVariableValue *value;
+
+  stack = g_array_new (FALSE, FALSE, sizeof (int));
+
+  g_hash_table_iter_init (&iter, self->variables);
+
+  while (g_hash_table_iter_next (&iter, &id, (gpointer) &value))
+    {
+      /* Have 2 copies of self->variables: "variables" can be iterated safely as
+       * we remove values, and "unvisited_variables" as a visited node marker
+       * for DFS */
+      g_hash_table_insert (variables, id, value);
+      g_hash_table_add (unvisited_variables, id);
+    }
+
+  g_hash_table_iter_init (&iter, variables);
+
+  while (g_hash_table_iter_next (&iter, &id, (gpointer) &value))
+    {
+      if (!g_hash_table_contains (unvisited_variables, id))
+        continue;
+
+      if (!g_hash_table_contains (self->variables, id))
+        continue;
+
+      if (check_variable (self, unvisited_variables, stack, GPOINTER_TO_INT (id), value))
+        {
+          int i;
+
+          /* Found a cycle, remove the offending variables */
+          for (i = stack->len - 2; i >= 0; i--)
+            {
+              int id_to_remove = g_array_index (stack, int, i);
+
+              g_hash_table_remove (self->variables, GINT_TO_POINTER (id_to_remove));
+
+              if (g_array_index (stack, int, i) == g_array_index (stack, int, stack->len - 1))
+                break;
+            }
+
+          g_array_remove_range (stack, 0, stack->len);
+        }
+    }
+
+  g_array_unref (stack);
+  g_hash_table_unref (variables);
+  g_hash_table_unref (unvisited_variables);
+}
+
+GtkCssVariableValue *
+gtk_css_variable_set_lookup (GtkCssVariableSet  *self,
+                             int                 id,
+                             GtkCssVariableSet **source)
+{
+  GtkCssVariableValue *ret = g_hash_table_lookup (self->variables, GINT_TO_POINTER (id));
+
+  if (ret)
+    {
+      if (source)
+        *source = self;
+      return ret;
+    }
+
+  if (self->parent)
+    return gtk_css_variable_set_lookup (self->parent, id, source);
+
+  if (source)
+    *source = NULL;
+
+  return NULL;
+}
+
+static int
+compare_ids (gconstpointer a, gconstpointer b, gpointer user_data)
+{
+  GtkCssCustomPropertyPool *pool = user_data;
+  int id1 = GPOINTER_TO_INT (*((gconstpointer *) a));
+  int id2 = GPOINTER_TO_INT (*((gconstpointer *) b));
+  const char *name1, *name2;
+
+  name1 = gtk_css_custom_property_pool_get_name (pool, id1);
+  name2 = gtk_css_custom_property_pool_get_name (pool, id2);
+
+  return strcmp (name1, name2);
+}
+
+GArray *
+gtk_css_variable_set_list_ids (GtkCssVariableSet *self)
+{
+  GtkCssCustomPropertyPool *pool = gtk_css_custom_property_pool_get ();
+  GArray *ret = g_array_new (FALSE, FALSE, sizeof (int));
+  GHashTableIter iter;
+  gpointer id;
+
+  g_hash_table_iter_init (&iter, self->variables);
+
+  while (g_hash_table_iter_next (&iter, &id, NULL))
+    g_array_append_val (ret, id);
+
+  g_array_sort_with_data (ret, compare_ids, pool);
+
+  return ret;
+}
+
+gboolean
+gtk_css_variable_set_equal (GtkCssVariableSet *set1,
+                            GtkCssVariableSet *set2)
+{
+  GHashTableIter iter;
+  gpointer id;
+  GtkCssVariableValue *value1;
+
+  if (set1 == set2)
+    return TRUE;
+
+  if (set1 == NULL || set2 == NULL)
+    return FALSE;
+
+  if (g_hash_table_size (set1->variables) != g_hash_table_size (set2->variables))
+    return FALSE;
+
+  if (set1->parent != set2->parent)
+    return FALSE;
+
+  g_hash_table_iter_init (&iter, set2->variables);
+
+  while (g_hash_table_iter_next (&iter, &id, NULL))
+    {
+      if (!g_hash_table_contains (set1->variables, id))
+        return FALSE;
+    }
+
+  g_hash_table_iter_init (&iter, set1->variables);
+
+  while (g_hash_table_iter_next (&iter, &id, (gpointer) &value1))
+    {
+      GtkCssVariableValue *value2 = g_hash_table_lookup (set2->variables, id);
+
+      if (value2 == NULL)
+         return FALSE;
+
+      if (!gtk_css_variable_value_equal (value1, value2))
+         return FALSE;
+    }
+
+  return TRUE;
+}
diff --git a/gtk/gtkcssvariablesetprivate.h b/gtk/gtkcssvariablesetprivate.h
new file mode 100644
index 0000000000..39bd1d4bde
--- /dev/null
+++ b/gtk/gtkcssvariablesetprivate.h
@@ -0,0 +1,60 @@
+/*
+ * Copyright (C) 2023 GNOME Foundation Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authors: Alice Mikhaylenko <alicem@gnome.org>
+ */
+
+#pragma once
+
+#include "gtk/css/gtkcssvariablevalueprivate.h"
+
+G_BEGIN_DECLS
+
+typedef struct _GtkCssVariableSet GtkCssVariableSet;
+
+struct _GtkCssVariableSet
+{
+  int ref_count;
+
+  GHashTable *variables;
+  GtkCssVariableSet *parent;
+};
+
+GtkCssVariableSet *  gtk_css_variable_set_new            (void);
+
+GtkCssVariableSet *  gtk_css_variable_set_ref            (GtkCssVariableSet   *self);
+void                 gtk_css_variable_set_unref          (GtkCssVariableSet   *self);
+
+GtkCssVariableSet *  gtk_css_variable_set_copy           (GtkCssVariableSet   *self);
+
+void                 gtk_css_variable_set_set_parent     (GtkCssVariableSet   *self,
+                                                          GtkCssVariableSet   *parent);
+
+void                 gtk_css_variable_set_add            (GtkCssVariableSet   *self,
+                                                          int                  id,
+                                                          GtkCssVariableValue *value);
+void                 gtk_css_variable_set_resolve_cycles (GtkCssVariableSet   *self);
+
+GtkCssVariableValue *gtk_css_variable_set_lookup         (GtkCssVariableSet   *self,
+                                                          int                  id,
+                                                          GtkCssVariableSet  **source);
+
+GArray *             gtk_css_variable_set_list_ids       (GtkCssVariableSet   *self);
+
+gboolean             gtk_css_variable_set_equal          (GtkCssVariableSet   *set1,
+                                                          GtkCssVariableSet   *set2);
+
+G_END_DECLS
diff --git a/gtk/meson.build b/gtk/meson.build
index 436a167ff0..db3c553d37 100644
--- a/gtk/meson.build
+++ b/gtk/meson.build
@@ -53,6 +53,7 @@ gtk_private_sources = files([
   'gtkcsscalcvalue.c',
   'gtkcsscolorvalue.c',
   'gtkcsscornervalue.c',
+  'gtkcsscustompropertypool.c',
   'gtkcssdimensionvalue.c',
   'gtkcssdynamic.c',
   'gtkcsseasevalue.c',
@@ -84,6 +85,7 @@ gtk_private_sources = files([
   'gtkcssnumbervalue.c',
   'gtkcsspalettevalue.c',
   'gtkcsspositionvalue.c',
+  'gtkcssreferencevalue.c',
   'gtkcssrepeatvalue.c',
   'gtkcssselector.c',
   'gtkcssshadowvalue.c',
@@ -101,6 +103,7 @@ gtk_private_sources = files([
   'gtkcsstypes.c',
   'gtkcssunsetvalue.c',
   'gtkcssvalue.c',
+  'gtkcssvariableset.c',
   'gtkcsswidgetnode.c',
   'gtkdrop.c',
   'gtkfilechooserentry.c',