In the multiplayer VS mode, selecting the New Magma City theme has it play Nice Day for a Stroll despite the background being that same unfitting night sky that Waste Disposal Plant had. I'm guessing this is a holdover from DDG1 where the city had a day sky by default, so maybe you could swap it for the day sky, or swap the theme for Chemplant like you did before, or add an extra option so you can do both with New Magma City (day) and New Magma City (night).