Do you want themes based on color or foods that are "scary" looking? You can set jello in straws and they come out looking disgustingly like worms (especially if you put a little cream into the liquid jello).
Also, one of my favorites, is making meatloaf into the shape of a hand with onion slices for finger nails. It was a hit at my dinner party.
I also made deviled eggs with cut up olives for spider legs and I had primal pizza with slices of mozzarella cheese cheese to make it looks like a zombie (with olive eyes).
There are lots of recipes out there if you look around!