Interestingly enough, i'm working on a similar idea for my site 😉 I want to be able to give users "skins" for the site. So that they can pick what they site looks like. However, it is more then simple CCS. It would be different graphics as well.
I haven't given it too much tought yet, but even though I could just use a bunch of if's that detect the cookie that tells me what Skin to use, that would slow down the site a lot i belive. The best approach I can think of is to have a set a dir called "skins" and in it named dirs for each skin. Then, on each main file on the site, call as an include the required Skin file....
Again, i havent looked into it much or even written any code, but I too would be interested to hear what the rest of you think.