This wiki is currently using a theme known as Monaco, which is the standard theme for wikia wikis and the only theme that you can edit the CSS(thus making a custom theme of it). However, come November 3, 2010, wikia will remove the entirety of the monaco theme and there will be a new standard theme.
Here's a wiki using the new standard theme: http://healthyrecipes.wikia.com/wiki/Healthy_Recipes_Wiki (NOTE: Sometimes clicking the link takes you to the monaco version of the site rather than show off the new theme)
Can you imagine this one using that theme? Neither can I. It's not bad but it doesn't even look like a wiki anymore. Sure, I'll probably make a custom theme of it, all red, gray and madness combatsy, but I am seriously considering leaving wikia and just creating the wiki on my server.