You can only start your project once the jam starts, at that time the theme will be announced
Viewing post in Do I start creating my game now?
You can't make a game you don't know the theme for, but you can prep. Like batman, with preptime he beats everyone. You can make input, UI, character, camera, etc controllers and other helper logic and focus on making the actual gameplay for the jam. It's completely unrealistic to do all that in 48hs AND the actual game. Tools =/= game.