gui: Fix dark theme help button

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3130

gui/default/index.html

@@ -50,7 +50,7 @@
         </li>
         <li class="dropdown" language-select></li>
         <li>
-          <a href="https://docs.syncthing.net/intro/gui.html" target="_blank">
+          <a class="navbar-link" href="https://docs.syncthing.net/intro/gui.html" target="_blank">
             <span class="fa fa-question-circle"></span>&nbsp;<span  class="hidden-xs" translate>Help</span>
           </a>
         </li>