diff --git a/core/theme.js b/core/theme.js
index 75bfb3779..16289a3ad 100644
--- a/core/theme.js
+++ b/core/theme.js
@@ -9,7 +9,8 @@
  */
 'use strict';
 
-goog.provide('Blockly.Theme');
+goog.module('Blockly.Theme');
+goog.module.declareLegacyNamespace();
 
 goog.require('Blockly.registry');
 goog.require('Blockly.utils');
@@ -19,16 +20,16 @@ goog.require('Blockly.utils.object');
 /**
  * Class for a theme.
  * @param {string} name Theme name.
- * @param {!Object<string, Blockly.Theme.BlockStyle>=} opt_blockStyles A map
+ * @param {!Object<string, Theme.BlockStyle>=} opt_blockStyles A map
  *     from style names (strings) to objects with style attributes for blocks.
- * @param {!Object<string, Blockly.Theme.CategoryStyle>=} opt_categoryStyles A
+ * @param {!Object<string, Theme.CategoryStyle>=} opt_categoryStyles A
  *     map from style names (strings) to objects with style attributes for
  *     categories.
- * @param {!Blockly.Theme.ComponentStyle=} opt_componentStyles A map of Blockly
+ * @param {!Theme.ComponentStyle=} opt_componentStyles A map of Blockly
  *     component names to style value.
  * @constructor
  */
-Blockly.Theme = function(name, opt_blockStyles, opt_categoryStyles,
+const Theme = function(name, opt_blockStyles, opt_categoryStyles,
     opt_componentStyles) {
 
   /**
@@ -39,32 +40,32 @@ Blockly.Theme = function(name, opt_blockStyles, opt_categoryStyles,
 
   /**
    * The block styles map.
-   * @type {!Object<string, !Blockly.Theme.BlockStyle>}
+   * @type {!Object<string, !Theme.BlockStyle>}
    * @package
    */
   this.blockStyles = opt_blockStyles || Object.create(null);
 
   /**
    * The category styles map.
-   * @type {!Object<string, Blockly.Theme.CategoryStyle>}
+   * @type {!Object<string, Theme.CategoryStyle>}
    * @package
    */
   this.categoryStyles = opt_categoryStyles || Object.create(null);
 
   /**
    * The UI components styles map.
-   * @type {!Blockly.Theme.ComponentStyle}
+   * @type {!Theme.ComponentStyle}
    * @package
    */
   this.componentStyles = opt_componentStyles ||
-    (/** @type {Blockly.Theme.ComponentStyle} */ (Object.create(null)));
+    (/** @type {Theme.ComponentStyle} */ (Object.create(null)));
 
   /**
    * The font style.
-   * @type {!Blockly.Theme.FontStyle}
+   * @type {!Theme.FontStyle}
    * @package
    */
-  this.fontStyle = /** @type {Blockly.Theme.FontStyle} */ (Object.create(null));
+  this.fontStyle = /** @type {Theme.FontStyle} */ (Object.create(null));
 
   /**
    * Whether or not to add a 'hat' on top of all blocks with no previous or
@@ -87,7 +88,7 @@ Blockly.Theme = function(name, opt_blockStyles, opt_categoryStyles,
  *            hat:string
  *          }}
  */
-Blockly.Theme.BlockStyle;
+Theme.BlockStyle;
 
 /**
  * A category style.
@@ -95,7 +96,7 @@ Blockly.Theme.BlockStyle;
  *            colour:string
  *          }}
  */
-Blockly.Theme.CategoryStyle;
+Theme.CategoryStyle;
 
 /**
  * A component style.
@@ -118,7 +119,7 @@ Blockly.Theme.CategoryStyle;
  *            replacementGlowOpacity:?number
  *          }}
  */
-Blockly.Theme.ComponentStyle;
+Theme.ComponentStyle;
 
 /**
  * A font style.
@@ -128,32 +129,32 @@ Blockly.Theme.ComponentStyle;
  *            size:?number
  *          }}
  */
-Blockly.Theme.FontStyle;
+Theme.FontStyle;
 
 /**
  * Gets the class name that identifies this theme.
  * @return {string} The CSS class name.
  * @package
  */
-Blockly.Theme.prototype.getClassName = function() {
+Theme.prototype.getClassName = function() {
   return this.name + '-theme';
 };
 
 /**
  * Overrides or adds a style to the blockStyles map.
  * @param {string} blockStyleName The name of the block style.
- * @param {Blockly.Theme.BlockStyle} blockStyle The block style.
+ * @param {Theme.BlockStyle} blockStyle The block style.
 */
-Blockly.Theme.prototype.setBlockStyle = function(blockStyleName, blockStyle) {
+Theme.prototype.setBlockStyle = function(blockStyleName, blockStyle) {
   this.blockStyles[blockStyleName] = blockStyle;
 };
 
 /**
  * Overrides or adds a style to the categoryStyles map.
  * @param {string} categoryStyleName The name of the category style.
- * @param {Blockly.Theme.CategoryStyle} categoryStyle The category style.
+ * @param {Theme.CategoryStyle} categoryStyle The category style.
 */
-Blockly.Theme.prototype.setCategoryStyle = function(categoryStyleName,
+Theme.prototype.setCategoryStyle = function(categoryStyleName,
     categoryStyle) {
   this.categoryStyles[categoryStyleName] = categoryStyle;
 };
@@ -164,7 +165,7 @@ Blockly.Theme.prototype.setCategoryStyle = function(categoryStyleName,
  * @param {string} componentName The name of the component.
  * @return {?string} The style value.
  */
-Blockly.Theme.prototype.getComponentStyle = function(componentName) {
+Theme.prototype.getComponentStyle = function(componentName) {
   const style = this.componentStyles[componentName];
   if (style && typeof style == 'string' &&
       this.getComponentStyle(/** @type {string} */ (style))) {
@@ -178,16 +179,16 @@ Blockly.Theme.prototype.getComponentStyle = function(componentName) {
  * @param {string} componentName The name of the component.
  * @param {*} styleValue The style value.
 */
-Blockly.Theme.prototype.setComponentStyle = function(componentName,
+Theme.prototype.setComponentStyle = function(componentName,
     styleValue) {
   this.componentStyles[componentName] = styleValue;
 };
 
 /**
  * Configure a theme's font style.
- * @param {Blockly.Theme.FontStyle} fontStyle The font style.
+ * @param {Theme.FontStyle} fontStyle The font style.
 */
-Blockly.Theme.prototype.setFontStyle = function(fontStyle) {
+Theme.prototype.setFontStyle = function(fontStyle) {
   this.fontStyle = fontStyle;
 };
 
@@ -196,7 +197,7 @@ Blockly.Theme.prototype.setFontStyle = function(fontStyle) {
  * @param {boolean} startHats True if the theme enables start hats, false
  *     otherwise.
 */
-Blockly.Theme.prototype.setStartHats = function(startHats) {
+Theme.prototype.setStartHats = function(startHats) {
   this.startHats = startHats;
 };
 
@@ -204,16 +205,16 @@ Blockly.Theme.prototype.setStartHats = function(startHats) {
  * Define a new Blockly theme.
  * @param {string} name The name of the theme.
  * @param {!Object} themeObj An object containing theme properties.
- * @return {!Blockly.Theme} A new Blockly theme.
+ * @return {!Theme} A new Blockly theme.
 */
-Blockly.Theme.defineTheme = function(name, themeObj) {
-  const theme = new Blockly.Theme(name);
+Theme.defineTheme = function(name, themeObj) {
+  const theme = new Theme(name);
   let base = themeObj['base'];
   if (base) {
     if (typeof base == "string") {
       base = Blockly.registry.getObject(Blockly.registry.Type.THEME, base);
     }
-    if (base instanceof Blockly.Theme) {
+    if (base instanceof Theme) {
       Blockly.utils.object.deepMerge(theme, base);
       theme.name = name;
     }
@@ -233,3 +234,5 @@ Blockly.Theme.defineTheme = function(name, themeObj) {
 
   return theme;
 };
+
+exports = Theme;
diff --git a/tests/deps.js b/tests/deps.js
index de575339f..42d14c6a8 100644
--- a/tests/deps.js
+++ b/tests/deps.js
@@ -161,7 +161,7 @@ goog.addDependency('../../core/requires.js', ['Blockly.requires'], ['Blockly', '
 goog.addDependency('../../core/scrollbar.js', ['Blockly.Scrollbar', 'Blockly.ScrollbarPair'], ['Blockly.Events', 'Blockly.Touch', 'Blockly.browserEvents', 'Blockly.utils', 'Blockly.utils.Coordinate', 'Blockly.utils.Metrics', 'Blockly.utils.Svg', 'Blockly.utils.dom']);
 goog.addDependency('../../core/shortcut_items.js', ['Blockly.ShortcutItems'], ['Blockly.Gesture', 'Blockly.ShortcutRegistry', 'Blockly.utils.KeyCodes']);
 goog.addDependency('../../core/shortcut_registry.js', ['Blockly.ShortcutRegistry'], ['Blockly.utils.KeyCodes', 'Blockly.utils.object']);
-goog.addDependency('../../core/theme.js', ['Blockly.Theme'], ['Blockly.registry', 'Blockly.utils', 'Blockly.utils.object']);
+goog.addDependency('../../core/theme.js', ['Blockly.Theme'], ['Blockly.registry', 'Blockly.utils', 'Blockly.utils.object'], {'lang': 'es6', 'module': 'goog'});
 goog.addDependency('../../core/theme/classic.js', ['Blockly.Themes.Classic'], ['Blockly.Theme']);
 goog.addDependency('../../core/theme/zelos.js', ['Blockly.Themes.Zelos'], ['Blockly.Theme']);
 goog.addDependency('../../core/theme_manager.js', ['Blockly.ThemeManager'], ['Blockly.Theme']);