Add .navbar-default.

docs/_includes/navigation.html

@@ -1,4 +1,4 @@
-<header class="s2-docs-nav navbar navbar-static-top" id="top" role="banner">
+<header class="s2-docs-nav navbar navbar-default navbar-static-top" id="top" role="banner">
   <div class="container">
     <div class="navbar-header">
       <button class="navbar-toggle collapsed" type="button" data-toggle="collapse" data-target=".select2-navbar-collapse">