How to apply wallpaper over styrofoam?
You should not do this.  Foam insulation (EPS, XPS, etc.) needs to be covered with drywall in order to protect it (extend the amount of time before it melts) from fire. Otherwise you are risk of being exposed to toxic fumes and melting foam should you ever have a fire. Imagine molten foam dripping from your ceiling onto you - not a situation you want to find yourself in.

The correct "solution" is to drywall on top of the foam (using furring strips, or long screws into the studs), mud/tape the drywall, prime, and then apply wall paper using standard wall paper adhesives.