Wew the custom backgroud button is still there? That's a silly bug ahahha thank you for letting me know.
To reset the background to default, navigate to:
C:\Users\YourUserName\AppData\Roaming\Godot\app_userdata\Gazō Tuber
In that directory, I believe there are files called something like "BG.save" or something.
You need to delete em.