While it is a nice thought, I somehow have to disagree. Managing themes and keeping them up-to-date is quite much work, and the more themes the more work it is. Additionally, the community might want to create themes, so you have to provide a basic toolkit and an interface for that. And then you have to keep even the community-themes up-to-date...
I'd agree to such a feature if the SO-Engine would be OpenSource, because it would be highly logical to provide it as customizable as possible, and then you really could place the responsibility into the hands of the Community.
I think client-side solutions are the best ones for this, like Stylish in combination with Greasemonkey. It's already easily possible to re-skin the trilogy.
Edit: You might also want to look at these three links: