Theme options are much more limited in OS X than in 9. 9 had hundreds and hundreds of Kaleidoscope themes that changed every possible thing about your appearance, much more so that the few things that Windoze themes did.
I still use it on my OS 9 box. Have a look at
www.kaleidoscope.netIn OS X, I just replaced the default aqua interface with a custom grape one.
I have shitloads of beautiful 128x128 icons, but my dekstop is toally devoid of icons or anything. I have full access to everything I need with a right-click though. All by hard drives and home directory appear in a contextual menu, no matter what app I'm using. Quite nice.