add button to toggle the optional theme when viewing the docs; fixes #9764

docs/_layouts/default.html

@@ -50,6 +50,11 @@
           </div>
         </div>
       </div>
+      {% if page.slug == "css" or page.slug == "components" or page.slug == "js" %}
+        <div>
+          <button id="bs-theme-btn" type="button" class="btn btn-default">Activate Theme</button>
+        </div>
+      {% endif %}
 
     </div>