I've got it, but each template is constructed in a different way, so without customize job inside template core it will be a little bit hard to do this using only CSS.
I shown you the way, which is connected with Rockwall template div's areas.
Check here: gk_rockwall\layouts\default.php
But sure you can play also with background image, some people (webmasters) uses photo with white box in the middle of it.
P.s.
Use Firebug tool to check which ID or Class you have to change/ override.