It's good that you changed the prosilver style to have a forum border with rounded corners, leaving you to now be little more creative working on styling the page body background behind it. But I've noticed both smiley and Who Online (IP checking) pop-up boxes are making things difficult. Reason being, both pop-up boxes are only small in height and use same background class as forum.

Read this topic with images shown to explain things better.

https://area51.phpbb.com/phpBB/viewtopic.php?f=81&t=45921