The other day Unsanity released ShapeShifter. It is a theme changing app for OS X. Previously I had not meddled in theme changing since I had heard some horror stories about system files being corrupted and problems subsequently occured. However, Unsanity advertises ShapeShifter as a safe solution and it seems to be true. They do not modify any system files while changing the theme and you just turn it off and you're back to normal.
I've been using it for a day now and it has been great. I'm using the 'Milk' Theme and it's pretty cool. I like the lightness of it, but I think I may want to get a darker theme in the future. There are some cool new themes available but they all seem to be Panther only. One of these days I'll get Panther.
I'm using Milk with this too. I wanted a change from the smoothstripes theme I was using. I'm not planning on getting Panther for a while, because Jaguar is satisfying everything i need at the moment.
Panther is worth the upgrade. I plan on buying Shapeshifter myself shortly. I haven't used it yet, but I love themes...