Not really - the theme will be announced at the start of the jam and may be drastically different to your planned game. Perhaps you can imagine some mechanics that are theme agnostic?
What can I work on before the jam? You are allowed to set up your tools and your game engine. Tools include: palettes, fonts, tracker instruments and code libraries.
so you could make some generic components that could be used in your jam game. like a basic character controller for a platformer and/or top-down game or a dialogue system. EDIT: sorry, i was mistaken!
these things would fall under core game programming and should not be worked on until the jam starts.
