Themes are implemented using CSS:
Based on the default CSS, you can write your own CSS, install it on the web somewhere, and use it for this wiki! See Specify a different CSS.
On the downside, whenever I change pages I get informed my cookie has reset itself to the default. Aaaaaargh!
So I noticed. Damn. Some script bug perhaps?
Is it assuming the css parameter is there, or perhaps a .htaccess file is putting it in?
This still seems to be conflicting with the rc cookie, though it may be this shoddy browser