OS X never had theme support and I doubt it will ever have. A big issue with themes is that they mess up UI design and such design-conscious company as Apple is certainly averse to that. And besides, Yosemite already offers quite a lot personalisation with the desktop picture affecting the overall appearance of the OS.
Personally, I was never fond of the "look" of OS X from 10.4 on, and like Yosemite over its predecessors - because it's different.
Given that, I actually liked the unreleased OS 8/9 themes that I got my hands on at the time from a friend - "Paper" and "Drawing Board", and I also used Kaleidoscope. What I don't miss is the instability in any system extensions from that era...
So, on that note, check out "Flavours" (http://flavours.interacto.net/). When you wrote "classic", I was thinking "Classic" as in the Classic Mac OS as it was called 10-odd years ago. Flavours isn't free, but I saw that they've got a Drawing Board theme and I think I'll pull the trigger on it. Cheers!