Fix custom-theme example to be more useful

Not it provides a meaningful example. It also just includes pre-built
css file for the example.

examples/custom-theme.html

@@ -6,7 +6,7 @@
   <link href="custom-theme.css" rel="stylesheet">
   <script src="../dist/js/perfect-scrollbar.js"></script>
   <style>
-    .contentHolder { position:relative; margin:0px auto; padding:0px; width: 600px; height: 400px; overflow: auto; }
+    .contentHolder { position: relative; margin:0px auto; padding:0px; width: 600px; height: 400px; overflow: hidden; }
     .contentHolder .content { background-image: url('./azusa.jpg'); width: 1280px; height: 720px; }
   </style>
 </head>
@@ -19,7 +19,7 @@
   var $ = document.querySelector.bind(document);
   window.onload = function () {
     Ps.initialize($('#Default'), {
-      theme: 'custom-theme'
+      theme: 'square'
     });
   };
 </script>