I've seen it done on other forums. I few years back I used to run a phpBB forum and it was simply a matter of downloading new themes and allowing the user to pick what they wanted.
I'm just speculating but I reckon vertical sports have chosen this theme and are sticking with it. One of the disadvantages of allowing users to switch themes is that is can mess some graphics up. For example if you were able to change to a yellow theme then non of the smilies would look right or the avatar that I created wouldn't look right because of the effect of the fist tearing through the background is blue.
When people are creating sigs for the forum they look at the colour scheme of the forum and make sure it fits, for example if someone had a sig that primarily had red colours in it, it would then look crap if the forum colours changed and matched the sig colours.