Yeah, maybe naming it Main Theme wasn't a smart idea. I made this thinking about the title music, I named the video like this just because "Main Menu" seemed kinda unattractive. My bad. I already have an idea for the main theme, a funky thing like the first game's one.
The music loops, in the video it fades away just because putting the plain file in wouldn't sound very nice to the listener.
I think that music should stay one for each level, and maybe change depending on situations such as your oxygen supply being almost finished, or a big swarm of enemies approaching the base. Still, I wanted to make the music more or less tense, or have different feels depending on how the level will be, making the alternatives I mentioned above unnecessary. Sound effects and drones should fix the remaining issues. Also, I have nearly finished two musics already: they are very atmospheric, with lots of melodies and sounds but put in a way that they aren't invasive. That's how I think this game's sountrack should be, energetic but very ambiencey.