hey, sorry it took me so long to respond!
The filenames for the custom levels are “byo_level_fg.png” “byo_level_bg.png” make them the same size as one another and place them in the RebopBlasters_Data directory (should be in the same place as the executable).
Unfortunately, only one player can play on the keyboard at a time because of how I set up auto-joining. It should support nearly every kind of controller though
Glad you enjoyed and sorry again for the very late response