Take a look at ShapeShifter:
http://www.macupdate.com/info.php/id/13335 - It's a 20 USD shareware that can apply a variety of themes you can download for free. The OS itself doesn't give you the ability to change much of the look of the GUI beyond what the Appearance preference in System Preferences offers...
I personally think, though, that Apple's done a great job with the look, and while themes may be 'interesting' for some time, ... I dunno. Take a look at them.
