I'm running up-to-date Ubuntu Gnome 14.04, and it's feeling pretty solid. However, I like to have a centered picture for my desktop, against a black background. This used to be easy to achieve with dconf-editor (org.gnome.desktop.background), but I've set the primary and secondary colors there to 0, but still the background appears as sort of off-white.

Where can I go to set the background color?

TIA...