Ten things you could be doing now
Get your itch.io page ready. Create the page, set up all the parameters, build some LD58 graphics. Setup your html to make it look nice. You can do a lot before you have a theme.
Decide on a game genre, platformer, top down scroller, side scroller, 2D or 3D. You can make those decisions now and then just plug in the theme.
Setup your project. Whatever engine, or framework you are using set the project up in the genre you are going to use. Get your screens set up and working. Block out the game and then substitute the graphics, sound effects, and music when the theme is announced.
Setup your game title screen. Use a generic screen and place your buttons for 'Start', 'Exit', 'Rules', and 'Play'.
Setup you Credits page that can be accessed from the game through a hot key and then you can return.
Set up your End Screen page buttons with 'Exit' and 'Play Again'
Setup your Rules Page, how the game is played. Make it simple
Optional, a 'High Score' page that can be accessed from the 'End Screen' page
If you are not making your own Music and SoundFX then make sure all the links for you assets are working for your SoundFX and Music. Freesound is a great place for sound FX
If you are not going to do your own graphics then find a place that has a ton of free assets, Kenney for 2D and Kay Kit for 3D are very good examples.
Finally. When the theme comes out keep your project small. REALLY SMALL!. For example, if the theme is 'Doors' then have maybe three levels of a rogue like game with doors that open only when all the treasure is found. Maybe have some enemies.
Stay away from inventory systems, RPGs, ARPG, games where you have to build up a reputation that takes time. Most of the developers on here are not going to spend more than 10 - 15 minutes playing a game because there are so many games to play, and your 'karma'\page rank depends upon the number of games you play. So make your game so that it can be played to completion or at least the first level in less that 15 minutes.
This is a game jam, a 48 hour game jam, and not standard game development. You goal is not to write your dream game, your goal is to finish something with a little polish that gives a few moments enjoyment to your player.
Hope this helps...