MD Settings: allow changing to GTK+ theme on Linux

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

Index: chrome/browser/resources/settings/appearance_page/appearance_page.html
diff --git a/chrome/browser/resources/settings/appearance_page/appearance_page.html b/chrome/browser/resources/settings/appearance_page/appearance_page.html
index 54e94194c18683aa4b0b032d18f61ac1fe78ffa2..c59777a4231cc6465ccc11bae61fc57e4af01639 100644
--- a/chrome/browser/resources/settings/appearance_page/appearance_page.html
+++ b/chrome/browser/resources/settings/appearance_page/appearance_page.html
@@ -24,6 +24,10 @@
           font-weight: inherit;
         };
       }
+
+      .secondary-button ~ .secondary-button {
+        -webkit-margin-start: 12px;
+      }
     </style>
     <settings-animated-pages id="pages" section="appearance">
       <neon-animatable route-path="default">
@@ -46,19 +50,39 @@
 </if>
           <div class="start two-line" on-tap="openThemesGallery_">
             <div class="flex">
-              <div>$i18n{theme}</div>
+              <div>$i18n{themes}</div>
               <div class="secondary">[[themeSublabel_]]</div>
             </div>
             <button class="icon-external" is="paper-icon-button-light"></button>
           </div>
-          <template is="dom-if" if="[[allowResetTheme_]]">
+<if expr="not is_linux or chromeos">
+          <template is="dom-if" if="[[prefs.extensions.theme.id.value]]">
             <div class="secondary-action">
-              <paper-button id="resetTheme" on-tap="resetTheme_"
+              <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">
+            <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_"
+                  class="secondary-button">
+                $i18n{useSystemTheme}
+              </paper-button>
+            </template>
+          </div>
+</if>
         </div>
         <div class="settings-box"
             hidden="[[!pageVisibility.homeButton]]">