Improvements and fixes to OB6 test

* Fix special page namespace
* Add grey background overlay
* Fix margin
* Change centering approach
** Works in IE8+ and modern browsers, shows in upper left in older ones
* Formatting fix to GettingStarted.hooks.php

Change-Id: Id9cc1d86d2da5e9c717b660aa3a3de9ec3ff0e1b