I wonder if I can start making the game now when the theme is revealed. Or should I wait and focus only on game planning? 

You can game-plan, plan things on paper etc right now. Unfortunately, no code, art, or music can be made for the game yet. Hold on, 16 more hours! :D By the way, the FAQ thread may be better for your question :)

Last reply is correct (including the FAQ thread usage)! From the rules on the jam page: 

• You can brainstorm, plan, and outline before the jam. However, any assets used in the game should be created within the jam timeframe.