How to finish/trim prefab fireplace
Each fireplace manufacturer has specific clearances and installation instructions. If you don’t know the brand, I’d get an installation manual like this one, and study it.

http://www.kozyheat.com/wp-content/uploads/2015/11/Z42-Z42CD-Manual-1-09.pdf

Also, I’d be concerned about the styrofoam exposed to the fireplace box. The fireplace gives off a lot of heat and it could affect the styrofoam. 

Also, if you use tile or rock to cover the face of the fireplace, use “fire resistant” type mortar, grout, etc. Otherwise, when the mortar/grout gets hot, it could all sag.