I like the site being centered biggrin
I know we were told not to take the background into account, but I'd like it much better if that cute art stretched the length of the page, or better yet if the image was stationary while you scrolled the page (for pages with exceptionally long posts...?) I've seen that kind of coding on profiles, my profile is like that in fact. So that might be neat!
Also, if we could select backgrounds, that'd be cool too (For instance, in the labs, there's a background downloader thingy... maybe you could combine that with this and allow people to set their own backgrounds from the numerous arts already available for users to download? :3)
As far as the text bubbles for forums is concerned... that's an easy fix, simply make the white boxes around the bubbles themselves transparent... so that only the bubble is visible against the background, and if people absolutely can't stand the white bubbles, perhaps make the backgrounds apply a 'theme' to the bubbles too, and give them a pastel tint that blends well with the background color.
There's a lot you could do with this feature to make the website more aesthetically pleasing! ^-^ I really like this feature a lot and hope you guys implement it!!