MD Settings: don't allow supervised users to switch to GTK+ theme

chrome/browser/resources/settings/appearance_page/appearance_page.js

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * 'settings-appearance-page' is the settings page containing appearance
  * settings.
  *
  * Example:
  *
@@ skipping 171 matching lines @@
     return !!themeId || useSystemTheme;
   },
 
   /**
    * @param {string} themeId
    * @param {boolean} useSystemTheme
    * @return {boolean} Whether to show the "USE GTK+" button.
    * @private
    */
   showUseSystem_: function(themeId, useSystemTheme) {
-    return !!themeId || !useSystemTheme;
+    return (!!themeId || !useSystemTheme) && !this.browserProxy_.isSupervised();
+  },
+
+  /**
+   * @param {string} themeId
+   * @param {boolean} useSystemTheme
+   * @return {boolean} Whether to show the secondary area where "USE CLASSIC"
+   *     and "USE GTK+" buttons live.
+   * @private
+   */
+  showThemesSecondary_: function(themeId, useSystemTheme) {
+    return this.showUseClassic_(themeId, useSystemTheme) ||
+           this.showUseSystem_(themeId, useSystemTheme);
   },
 
   /** @private */
   onUseSystemTap_: function() {
     this.browserProxy_.useSystemTheme();
   },
 </if>
 
   /**
    * @param {string} themeId
@@ skipping 31 matching lines @@
 
   /**
    * @param {boolean} bookmarksBarVisible if bookmarks bar option is visible.
    * @return {string} 'first' if the argument is false or empty otherwise.
    * @private
    */
   getFirst_: function(bookmarksBarVisible) {
     return !bookmarksBarVisible ? 'first' : '';
   }
 });