Where are sporecasts or themes stored on the PC? I want to make one out-of-game.

I want to try to make one by manually putting pngs in the folder. Literally the only thing I want to do is make a theme specific for a certain save file, but the only way to do that is through sporecasts, and some of the animals have too many parts, which I can't get rid of. If there's a way to make a theme out of game with whatever creatures I want, then PLEASE tell me.