So, after a lot of trial and error (and swearing), I believe I have successfully managed to implement user-selectable styles for the site, even including the forums. So far, there are only two, but I may make more in the future if there are requests for specific changes. To select a style, go to the main page of the site and use the widget on the top of the left-hand sidebar. Just click the link, and that should do it for you. Your selection is stored as a cookie and should persist across sessions, as long as you’re using the same browser.
As always, let me know if you encounter any bugs, especially in the forums after this change. The css interactions were complex, and annoying. I think I’ve worked around all the major issues that came up, but I can’t be sure I’ve found everything just yet.