Dark uses less power because it doesn’t have to work as hard to brighten the screen.
Light text on dark background is better for the eyes. Dark on light became the standard though because its easier to make black ink and white paper than white ink and dark paper.
This is only true for OLED screens. On an LCD, it technically uses slightly more power on dark to put the black “ink” in front of the white panel.
Light text and dark background on paper become hard to read in dim lighting or as you age. You want the lightness of the paper to reflect light.
I find that a bright office with windows needs a light theme for computer screen and, a darker office is suited to dark theme.
This is also why I judge light mode advocates - I think they’re still printing out copies of webpages. lol.