W32: Add .dim-label style

https://bugzilla.gnome.org/show_bug.cgi?id=727245

gtk/resources/theme/gtk-win32-base.css

@@ -1331,3 +1331,9 @@ GtkPopover > .toolbar {
 .list {
   background-color: -gtk-win32-color(listbox, 5);
 }
+
+/* Dim label */
+.dim-label {
+    color: alpha(currentColor, 0.55);
+    text-shadow: none;
+}