Go into level editor, press K to delete the existing background, and press F to generate a new one! While following the instructions obviously... really please just follow the instructions and don't type something like "obama"
This is obviously very far from perfect, but can still be a useful tool. I think. @griffpatch for the original Appel In case you care what code did I make then go into the "wallpaper" sprite :) there is a secret
