fix the popover bug in the app example by making the custom css more specific for .content

examples/container-app.html

@@ -29,7 +29,7 @@
       }
 
       /* The white background content wrapper */
-      .content {
+      .container > .content {
         background-color: #fff;
         padding: 20px;
         margin: 0 -20px; /* negative indent the amount of the padding to maintain the grid system */