I am right now working on a theme. Themes will be officially supported at some point, although right now the installation is not a trivial task. My theme is called simple and you can download it
Update: with the last Caffeinated update, Simple theme is working again! Download it at
http://cl.ly/Bm5k http://cl.ly/Cc4b http://cl.ly/D9kq
This should be the definite version of Caffeinated so theme definitions should not change again. It should be installed by double clicking on it. If not, just place the theme file into ~/Library/Application Support/Caffeinated/Themes directory
Please feel free to send me your comments about it.