You would be correct if someone was charging to design a model of the Enterprise, for example. However, what's being charged here is a modification of vBulletin, not modification of Star Trek material. That's like saying if someone has a Star Trek image on their forum, I couldn't pay them to modify my forum somehow.

Your analogy is a bit off. If you have a Star Trek image on your forum (eg: somebody posted it in the forums) and you pay somebody to modify your site then what Kayvin is talking about does not apply (you, on the other hand, might still be liable if you can't defend a fair use argument). However, if you were to pay somebody to design a custom theme for your site that includes a Star Trek image then copyrights do apply. It is very simple... a person (eg: a theme designer) can not sell images from Star Trek in their derived works unless they have a license from Paramount.

It generally covers original material themed on Star Trek too. Copyright doesn't necessarily mean an exact copy, otherwise you'd see plenty of things that are just slightly off to skirt copyright laws. Essentially, you're violating fair use if you profit off of the creation of anything pertaining to the Star Trek franchise that's not licensed.

Generally most of this sort of stuff they won't find out about, mind you, especially since a transaction like purchasing a custom theme is generally done privately. As long as you're not trying to sell a Star Trek-related theme openly and publicly, it's not a huge deal, especially considering a lot of themes wouldn't even be possible without paid stylists.

I bet everyone violates copyright law at least twice a day without realizing it. :D