stories.xnxx.com doesn't appear to have any content restrictions at all. I also can't find a way to change the background color.

bdsmlibrary.com is a much better organized site and fits most of your criteria(free, accepts rape/snuff/necrophilia, has a "all characters are 18 or over" rule that it doesn't seem to enforce), but the admins have been MIA since April and nobody knows if it'll ever be updated. Still, it's someplace to keep in mind.