common/static/common/styles/main.sass

@@ -9,6 +9,7 @@ $container-width: map-get($container-max-widths, 'xl')
 
 /* Web Assets. */
 @import '../../../../assets_shared/src/styles/main.sass'
+@import '../../../../assets_shared/src/styles/_theme_system.sass'
 
 /* Extension Platform specific styling. */
 @import '_mixins.sass'

common/templates/common/base.html

@@ -132,6 +132,10 @@
             </li>
             {% endblock nav-upload %}
 
+            <li>
+              <button class="js-toggle-theme-btn"><i class="js-toggle-theme-btn-icon i-adjust"></i></button>
+            </li>
+
             {% if user.is_authenticated %}
               <li class="nav-item dropdown">
                 <button id="navbarDropdown" aria-expanded="false" aria-haspopup="true" data-toggle-menu-id="nav-account-dropdown" role="button" class="nav-link dropdown-toggle js-dropdown-toggle">