For example changing Megalovania with Song That Might Play When You Fight Sans?

No, we don't allow any sort of modification of the game's files besides steam.api and data.win, for a number of reasons. First of all, it makes it so that the game you're playing is pretty much identical to game when first purchased. And second, allowing modifications to the audio or graphics of the game could potentially give people an unfair advantage over other runners.

Yeah, it was decided a while ago to disallow all game modifications as a standardisation/anti-cheating measure. Same with the colour mod, don't use it.

Okay, thanks guys!