Turntable-Themes is a project mainly written in ..., it's free.
Tools to make themes for Turntable.fm
All the info you need to make and use your own theme. There are two ways to use these themes, one is by bookmarklet, and one is by extension.
The bookmarklet method is quick and dirty. And it only works the first time you enter a room. You take the code you've written/copied, add "javascript:" in front of it, and make it a bookmark URL. Click the bookmark, theme applies. Special Note: Don't copy the "function" tag for a bookmarklet, just the content inside.
Soon there will be a "Custom" option on my Turntable Themes extension (Google it), and you will be able to put in a URL to a custom made theme. That custom theme will be a .js file and will apply your theme. Easy.