I liked the ancient rusted metal theme a great deal, it puts me into mind of an old abandoned building. The colors also remind me of old unpainted wood.
What about a theme, with that color pallet in mind, only with wood grain, and with windows with broken glass, or a door off the hinges, cob-webs spread across things.
Background images could be a side wall of the house itself. The forum buttons could be windows, or doors, you get my idea. Be a nice different Halloween that wouldn't be too dark.