I use Clearlook-Phenix and I like it. It is very neat and clean. It has been developed for GTK 3.0 to GTK 3.6.
I also am looking for XFWM themes. I'm using Paranoid GTK by Monkeymagico, which looks awesome, but frankly, all of the XFWM themes I've tried look so cheap next to the GTK. They all seem to be simple variations of the stock XFWM themes. Seems like XFWM is growing in popularity, but themes are disappointingly lacking. Any suggestions for a good dark, flat one?