As answered by the FAQ on the jam page:
Can I use premade assets, scripts, or ideas?
Yes! When it comes to assets you can use them as long as they follow these guidelines.
- The asset is general purpose. (Things like character controllers )
- You have the rights to use them.
- Write which assets were made before the jam in your description.
- Credit any assets you don't own in the description.
As for ideas, you are free to plan out your game before the jam starts, and also remember the theme!