Maybe give them a custom header image, like profiles have. But please don’t encourage them to fill my screen with #FF00FF.
Example:
This is it! We are a real user-eccentric social media!
People spending a looot of time to ruin perfectly good CSS in the most idiosyncratic ways so you can never look at the page without forgetting for even 1 single second that they did that and wondering WHY they were motivated to do so.
I am personally a HUGE fan. There is a big time collaboration between the ability to run wild in this way, and dynamic communities. Compare MySpace vs Facebook. Also on Team Bold Design: tumblr, livejournal and even reddit (after a big fight when they tried to take it away!).
I do also think you gotta have an option to hide all this beautiful wild stuff. Permanently and globally, for specific pages, and temporarily.