Well we already have the style chooser, which changes the forum style for someone. I don't particularly see why we couldn't add more styles if people wanted them, besides how well stuff like fit our custom tags/titles (w/e is the better word for it) for forum users would transition between different styles.
As for something to change imagery for specific parts of the forums (like the background, for example), if there's a plugin that allows users to do this for themselves then I'm unaware of it. I believe I talked to
@Laurenz about this though and he said it couldn't be done, so maybe this is a lost cause.