I'm performing a wedding on Halloween this year. It's on a farm, but I haven't heard if there is any theme yet. I'd love to be able to show up in costume for a wedding one of these years!
I've performed several weddings on Halloween over the years, can't think of any that had any Halloween decor. One October 31st couple didn't have any costume, or any dressing whatsoever, just their birthday suits. (I wore a tan dress.) We did it on a private beach in the late afternoon. Then they went skinny dipping to celebrate after. Nothing to do with Halloween, though I believe it was the anniversary of their first date.
There was one time, a bride gave the groom a plastic spider ring during the ceremony, since it was an October wedding, and he didn't want to wear a ring (for work) while she was given a big beautiful ring. That's the closest I've gotten to halloween costuming at a wedding, so far in my 12 years of performing marriages.