Why shouldn't someone be able to express themselves by having bright pink text on a bright rainbow-unicorn background? It's not my cup of tea, but I bet it makes a bunch of people happy.

The best solution, imho, would be to have user-styles enabled by default (these could be as ridiculous as people wanted), and then having a standard site-wide mechanism for reverting to the standard style (this would also shut down any background music or videos).