Stolzyboy, why is that silly? If you do it the way Mordecai suggested, then whenever you want to either a) change the look/feel of the site, or b) use different "skins" for your site, all it requires is a changed / new css file.
Otherwise, you'd have to go in and modify the php.
Check out phpbb's implementation of this concept. It's pretty well done, if not a little complex.