Mac OS X was a top-to-bottom change to the system architecture, and theming was infinitely more difficult. It took a long time before creative developers figured out a way to bring customization to OS X. There were themes, if only a few dozen, and of course you could still customize icons. But it was never to the extent that you could in Mac OS 9.
Eventually (I don’t remember if it was OS 10.4 or 10.5), theming became nearly impossible. But when Apple released the Mac App Store, customizing your Mac desktop all but died. Because of the code signing of all apps sold through the Mac App Store, altering files contained in individual apps (such as icons) rendered them either useless, or at the very least prevented you from updating them in the Mac App Store.
Between code signing, recently implemented Sandboxing rules, and the release of OS X Mountain Lion (which prevents theming of the Dock), it’s all but a dead art. If you need any more evidence, Panic Software recently announced they were sunsetting their icon customization tool, CandyBar. For many years, CandyBar was the gold-standard of customizing icons. Thankfully, Panic made CandyBar freely downloadable, and updated it for Mountain Lion. For those like me who used CandyBar for it’s icon collection organizing feature, and the ability to quickly and easily export app icons as PNG images with transparency intact, the fact that it still works is a bit of relief. But it’s future is most decidedly in doubt. It surely won’t be long before it can no longer customize System icons.
It’s sad to see theming and customization fade off into the sunset. But to be honest, Apple has improved the appearance of the OS to the point where even the most avid themed simply preferred the clean look of the standard theme. And right now, you can get an absolutely fantastic icon customizing and organizing app for free.