Two Words... PASSWORD PROTECTION.

VERY VERY VERY COOL, if you run it locally on your machine (which I suppose is your intention)... but lets say that a lot of people are forced to use win2k at work with inadequate services to do this (looks around, and stares at his feet).

Supposing that they wanted to run it off your site, someone could play a very nasty trick and give a user a real headache, or white out their screen.

You should keep it so that people can try on other people's themes though. Another idea, make it so that people can copy other peoples themes into theirs. Another idea would be allowing new wallpapers/images/other such things

Just a few suggestions.

"We're all different!"
"I'm not"
-The Life of Brian