What compounds are created by mixing Styrofoam and paint thinner?
Polystyrene will be dissolved in acetone. The long polymer molecules will dissociate from each other and float around in the acetone solvent; but the polymer chains will likely not be broken down into their small styrene monomer units. If you mix acetone and polystyrene ("styrofoam"), you will simply end up with polysterene in acetone. Not significant chemical reaction is expected to take place, only the physical process of dissolution/solvation/dissolving. 

I would assume such a solution is toxic and keep it far away from animals, eyes and mouths.