It should be pointed out that modern LCDs use local dimming zones to only light up certain parts of the display, although that only really helps if large swaths of the image are solid black. LCDs have come a long way from the old days when they were side or backlit by CCFLs. So even LCDs might draw slightly less power for light-on-dark, although you’d probably get even more benefit by just turning down the displays brightness regardless of the color scheme.