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

chrome/browser/resources/settings/appearance_page/appearance_page.html

 <link rel="import" href="chrome://resources/html/i18n_behavior.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-flex-layout/classes/iron-flex-layout.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/neon-animation/neon-animatable.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button-light.html">
 <link rel="import" href="/appearance_page/appearance_browser_proxy.html">
 <link rel="import" href="/controls/controlled_radio_button.html">
 <link rel="import" href="/controls/settings_dropdown_menu.html">
 <link rel="import" href="/controls/settings_input.html">
@@ skipping 49 matching lines @@
           <template is="dom-if" if="[[prefs.extensions.theme.id.value]]">
             <div class="secondary-action">
               <paper-button id="useDefault" on-tap="onUseDefaultTap_"
                   class="secondary-button">
                 $i18n{resetToDefaultTheme}
               </paper-button>
             </div>
           </template>
 </if>
 <if expr="is_linux and not chromeos">
-          <div class="secondary-action">
+          <div class="secondary-action" hidden="[[!showThemesSecondary_(
+            prefs.extensions.theme.id.value, useSystemTheme_)]]">
             <template is="dom-if" if="[[showUseClassic_(
                 prefs.extensions.theme.id.value, useSystemTheme_)]]" restamp>
               <paper-button id="useDefault" on-tap="onUseDefaultTap_"
                   class="secondary-button">
                 $i18n{useClassicTheme}
               </paper-button>
             </template>
             <template is="dom-if" if="[[showUseSystem_(
                 prefs.extensions.theme.id.value, useSystemTheme_)]]" restamp>
               <paper-button id="useSystem" on-tap="onUseSystemTap_"
@@ skipping 82 matching lines @@
             associated-control="[[$$('#customize-fonts-subpage-trigger')]]"
             page-title="$i18n{customizeFonts}">
           <settings-appearance-fonts-page prefs="{{prefs}}">
           </settings-appearance-fonts-page>
         </settings-subpage>
       </template>
     </settings-animated-pages>
   </template>
   <script src="appearance_page.js"></script>
 </dom-module>