Why not add a user_preference for an external style sheet? Mark up the forum pages with a number of classes and let users hack away at their own version of Nirvana? I know that you can do some of this with your browser setup, but it doesn't move from computer to computer or browser to browser so easily. Maybe store them in file-storage or somewhere easy to access. Trade and borrow style sheets with your friends!
The only issue with usability I noticed that hasn't been mentioned so far is the Forums Section in the banner. When you are reading a message or are in a specific forum, the "Forums" text is not a link, so the only way to get to "Forums" is to use the breadcrumb/context link. It seems like you could change the text color, but still preserve the link when you are below the index page.
