There's three classes, I'd call them, of TV themes.
First, there's songs borrowed for shows. "It's All Been Done" would fit this (theme to Baby Blues, but BNL wrote it way before). These just need the band.
Second is shows written for a show, but aren't related to it. "Having an Average Weekend" would fit here- I believe Shadowy Men wrote it for Kids in the Hall, but it's not about the show at all. Here, it could be like the first or the third case depending on who actually owns the material.
Last is songs written about the show, which are really tricky. The Carmen Sandiego theme, for example. You'd need the band/label's approval, and the trademark owner's approval (which is likely the show runners).
So this is why stuff like the Power Rangers theme is so complicated. There's more parties involved, including some that probably have no interest in promoting the music they've got control of. Which sucks sometimes, there's some awesome songs that've been used or written as TV themes.
Yeah, they can be tricky. Some themes just wouldn't work in the game anyways because they are 30 seconds, but some would be pretty awesome. I think getting "Believe It Or Not" by Joey Scarbury from "The Greatest American Hero" would be awesome, but no idea who to contact. I also just heard another song by him called "Again and Again" and it would be great in the game. I'd contact him, but it seems he released his album and other singles on about 7 different labels.
RBN/DLC Wishlist: Exit Ten, Deadlock, Dead Letter Circus, Foxy Shazam, Before the Dawn and Fair To Midland