Was this removed?
I am having trouble finding it.
That actually controls the dark black gradient you see around such as the nav bar, sub headings, sidebar ect. We use to use an image in old versions, this version uses a CSS3 gradient.
If you care to change that:
http://www.colorzilla.com/gradient-editor/
Is a good generator that will work fine. Again that effects all the black dark bars around the site.
Thanks, will play with that one day.