I agree with the posters that Apple is holding off on 'themes' in order to create brand recognition. I also think that Apple is holding off on 'themes' in order to enforce Trade-Dress trademekaing issues.
I know a lot of people believe that the turning point for Apple was the day it lost its case against Microsoft for copying/borrowing the windowing environment introduced in Windows 3x. It was a difficult case to justify and win. However, if Apple consistently and constantly pursues any and all violations of what it considers a threat to its trade-dress, its copyrights, its patenets,..etc, it makes it easier to enforce those very items in a court of law.
If Apple appears to allows violations or modifications to its 'stated' property, then they hold little validity in a court of law for the future.
Seems to me that Apple is trying to cover its own ass.
But, as a side note, I remember reading a while back (4-6months) that Apple had applied for a patent on a 'theme-ing' system. A questionable application, at best, but it seems that not only are they trying to enforce the issues I described above, but they may be getting set to allow themeing as long as they can still protect their rights and investments.
It the same with any item you purchase, really. I mean if you can figure out how to make the changes to OS X, feel free, it's yours, but they certainly don't have to support it or warrant it. Just as the proverbial car manufacturer doesn't have to warrant your corroded car because you decided to paint orange polka-dots on it.