Black pixels = off
Color/white pixels = on
Make the math.
But since Dark GAF isn't black, he wouldn't be saving battery, would he? Pixels would still be on.
It doesn't have to be black for it to help with battery savings.
OLED uses electricity to energize or charge the organic film to certain degrees. More electricity to fully charge a section to pure white, requiring less and less as you move towards darker colors.
Also, most OLED displays don't display pure black to begin with.
There is always a small charge applied to the display as it's apparently more energy efficient to step through voltages starting from a positive base, than a base of 0 volts. I don't think a phone has had true black levels since the first galaxy S. I could be wrong though.