Hey there Stephanie!!! Welcome to the WeddingWire community!!!
I love Halloween weddings. So much. I would try and stock up on Halloween decorations this year once everything goes on sale in November. You can use skulls as centerpieces, paint them purple or black or leave them creepy white. White pumpkins are also a thing that I've seen and I bet they'll be around and on sale this time of year. Have wedding colors like black, burgundy, royal purple, a nice orange, and dark blues. You could get a bunch of those plastic spiders and use them to decorate napkin rings. Or have a photo booth (or a head/sweetheart table) background with those fake cobwebs. Masks as favors, glow stick send off. The possibilities are endless.
There are also a whole bunch of discussions in the WeddingWire forums for ideas. I've included a couple of them below, but I would definitely search a little more if you have specific ideas and questions.
My venue had a Halloween themed wedding last year and our wedding coordinator said the couple named their tables after horror films and people were directed to their seats by those names. Not sure what else they did but I thought that was cute. Maybe you could do something similar?
There's been a couple halloween themed weddings posted here on wedding wire in the past. I'd search the forums and you can see how they executed it. It can be done so nicely without it coming across as a birthday or halloween party. Good luck! I love Halloween