If the background colour changes though, then the text colours may be harder to read. Like we've currently got a rule against the use of black text on the forums. If there is then a background on which black is the easiest to see, and say, default white is harder to see, what happens then? Also, logos would have to be redesigned, which would take a lot of work.
As a personal preference, I like the dark colours, I find them easier to see, but I can understand people wanting a change and some personalisation now and then.