OS X does not have anything that they identify as a theme, but you can change the look of the desktop and windows in the Appearance preference pane and the Desktop and Screen Saver preference pane within System Preferences. A google search did turn up some web sites that offer themes:
It doesn't look like there is the ability to change themes in Snow Leopard. After doing some digging on Google it looks like Magnifique (I think that is what it is called) doesn't work on Snow Leopard, but does work on Leopard. If you really want to have themes I think you'll have to look at Windows or Linux. I'm guessing that the next version of Mac OS X (Lion) will have a different theme than the current look of Snow Leopard, so you might just want to wait until then.