Does Stack Exchange offer themes (just like in gmail) where the user can choose his best liked theme for each site?
If not, how about having it?
Stack Exchange sites do not have native support for Themes, but many browsers support custom stylesheets that you can use to override the default CSS.
Instructions on how to do that here:
Or you can use a UserScript, as described here:
BUT, and that is a big but... You can use FireFox with GreaseMonkey to load some custom CSS for the sites. Happy hacking!