Can i make art assets like 2D sprite or 3D models before that the jam is started and then actually use them into the game?

I think everything has to be made when the jam starts. Including code, music, art etc..

Yes, you can. Exitent assets are allowed no matter who made them or when :)

Well i dont think is a good idea the theeme havent been realeased

This is my first Game Jam, but I have watched a few, and it seems that (on Unity at least) you are allowed to make the materials, so if you have a colour or texture that goes with anything, then maybe try that.

I on the other hand, will be doing no learning about the coding and stuff for another day or two... 

So expect a terrible game.