Believe it or not, this theme is actually a remix from Gremlins 2 on the NES, I was playing that game one day and the music struck me as familiar, when I made the connection I was pretty amused by that obscure little tidbit. This version is faster and a bit more crisp-sounding, but yeah, look up Gremlins 2 Office Theme and you'll hear where this originally came from :P
@Quadraxis14 I've posted the other themes and am now interested to know what NES games they're originally from. It'd be useful if someone could identify them for me.
@TheBananaNababa Hmm, well the end credits sound vaguely familiar but more in style than in actual theme, sort of Samurai Zombie Nationish, but I'm sure it's not from there. I think the opening and False Raiden songs at least are original, they don't have that retro NES sound compared to the other songs, but yeah, the level select, fight theme and end credits all sound like they could've come from NES games. Quite a mystery XD I guess the best person to ask would be the creator.