Right now the options are very limited. If you go to the Look and Layout Preferences in your Profile, you can change from the default theme, but the three other options you'll be given don't look much different from the default. There are some other themes available that I could upload and make user-optional, but using them would disable some addons (like being able to show YouTube videos) that only work with the default set.
When SMF 2.0 is releaseed (it's in beta now) I may be able to offer more choices without losing functions. We'll just have to wait and see.