Wrapping the entire forum in a DIV

I designed my site before coming across Xenforo as a forum choice (and may I say - WOW! rockin' forum software!!) and I'd like to wrap the whole thing in a DIV so I can show my site background and all and constrain the forums to a typical 960 width DIV. Any quick tips on how to achieve this or are you guys gonna make me work for it? If so, be warned, I have cookies!!