1. Remember what I said about keeping your posts short and simple. I suggest you don't make such a long reply chain in such a short time frame, but instead edit already existing reply to include what else you want to say to reduce clutter.
2. The theme will come once the jam starts; that's in 2 days so you'll have to wait until then. The only way you can prep yourself until then is by learning some general Godot knowledge. UI building, Scripting, Collision handling, etc. That way you can at least a basic game with any theme.