In the file named rhythm_game_displayable.rpy, look for the line that says add Solid('#000'). This adds the solid black background. Replace it with add "YOUR_FILE_NAME.png"
rhythm_game_displayable.rpy
add Solid('#000')
add "YOUR_FILE_NAME.png"
See here: https://github.com/RuolinZheng08/renpy-rhythm/blob/ff944b4f6efc189fd64ab68a56eab5b89fc380c9/game/00-renpy-rhythm/rhythm_game_displayable.rpy#L114
