The main menu background can be replaced, even the "Welcome" text that shows when you run the game. But I haven't seen a skin that replaces the "circles!" song that plays when you run the game. It could be cool if that can be changed.
PD: If there exist a skin that changes the opening song, let me know pls.
PPD: My english may can't be understandable sometimes. Let me know if you see a typo too.