There shouldn't be any background for the dialogue. Maybe it's something with the Android repacking process.. To be honest, RenPy is really complicated for android releases, I recommend using JoiPlay for mobile...View attachment 4157546 View attachment 4157547
Decent game, but you should remove that black shadow from the subtitles, especially in the Android version or playing on joiplay, you can't see any of the game's CG.
You can hide the dialogue bar by pressing H at any time.