Verified answer

Polystyrene will work just fine. The best thing to use to cover it is concrete floor leveler. Use a few coats and any rock work, etc will really pop. Add in some sand and rough it up a bit to make a natural look.

To paint it, acrylic craft paint is non toxic and will work just fine. Three different colors usually make convincing rock work. Use as many coats as you want to make it look good, this is the artistic part.

Another method is to use ACE Brand Concrete Binder/Primer (essentially Elmer's glue) mixed 1:1 with whatever you want the background to look like. Say you want to make a sandstone looking background... spread the mixture on, using sand as your additive. Anything works... bark, coco fiber, twigs, mulch. A coat of paint or floor leveler as underlayment first will make it look better.