Decorations:
Candlelight is just as lovely at a reception as at a ceremony. Candle arrangements, perhaps circled by dark flowers or ivy, make beautiful centerpieces.

Roses -- fresh or dried -- can be fashioned into a variety of centerpieces. Even a single dark blossom floating on water in a shallow glass dish can look stunning. Flower petals and confetti (maybe glitter!) add a pretty accent to dining tables.

The Wedding Cake:
Typical wedding cakes are predominently white, but most standard cake decorators can add colorful flowers. You could have rich red or purple roses (either frosting or real flowers) scattered around the cake, along with dark greenery. Or, if you find a more creative cake decorator, you can have spiderwebs and bats piped in frosting all over the cake.

For the cake topper, look for Nightmare Before Christmas figurines (Disney used to carry some; check eBay for second-hand figures) or Beetlejuice cartoon figures (these are no longer in production, but might be found on eBay as well). Other gothy figurines would be cats, bats, dragons, and fairies.

Or take traditional Bride and Groom caketop figures and give them a gothic makeover. Paint their clothes in dark colors, and paint on gothic makeup and hair. Or find two small dolls and sew gothic clothes for them. There are also many companies who will create custom portrait caketop figurines (altho' these are usually quite costly).

Music:
Dancing to DJed gothic music would be the perfect cap to a gothic wedding. If not all your guests are fans, you can mix the goth tunes with something a little more mainstream. A mix of gothic and 80s synthpop works well, as does gothic and 90s alternative rock.