I just got finished watching The Muppet at Walt Disney World Again. I'm even more convinced that "Bein Green" is more Kermit's personal theme. Kermit gets to sing "Rainbow Connection" with Raven-Symone, no problem. But when he attempts to sing "Bein Green" because it's a classic "swamp song", everyone groans and tells him to give it a rest!
So since the original question was, "What is Kermit's theme?" I think that answers the question for me.