I have not yet been able to find a theme I like. For anything.
I'm a big fan of light-on-dark. And simplicity; the more "cartoonish" icons (esp. the tray icons) do appeal to me. Not to say that gradients, blurs, 3d-curved looking effects, etc. are bad, just often highly over-used. One theme I do like is the Detonation-Green theme for XMMS/winamp:
http://xmms.org/files/Skins/images/detonation.jpg
If I had time, I'd love to make a Detonation-like theme for OS2008. Since I don't, I'll resort to begging: If anyone is looking for themes to make, and kinda likes that screenshot, please make me one!