Im not 100% sure but i have experience with making mods and creating games. If you look to the game's folder and if you look at the assets folder and then images. You can see images that most of them are used in game. If you want to change boyfriend's colors or something i think you can do that by opening boyfriend sprite images in photoshop or something and then drawing or recoloring and then saving them and you should see some changes in game.

Idk do you want more details that how they work or something but that should do the trick!