If you click on the "edit background" link at the top of one of your domus pages, insert the color # that you want in the "bgcolor" slot. In order to make the color show, however, you also have to remove the content from the box that says, "Please do not type a number in this field -[]-"

Now I always thought that meant "don't monkey with this box" but it seems to mean "if you want to change the background color, this box has to be empty."