Dark themes only save battery power on devices with displays that have per pixel lighting (eg oled like a modern iPhone). If the background lighting is fixed like an led or lcd display then the thing that impacts your battery is the brightness level, and most users can view a light theme at lower brightness than a dark theme.
If you're using dark mode to save battery on a laptop it's not working.
In fact on lcds black consumes a tiny bit more power because polarizing the lc foil takes a little bit of power. But I doubt you can measure the difference through the noise of normal computer operation.
On a phone, it has added roughly 4 hours of battery life at night. I'm not a big fan of dark theme, but only started using it for the battery conservation.
>If the background lighting is fixed like an led or lcd display
This hasn't been true of LCD displays for years. They now use LED backlighting, and the backlighting is dynamic (it varies in brightness depending on what'd displayed, and changes across the display too).
>If you're using dark mode to save battery on a laptop it's not working.
My business class just-retired-to-k3s-duty laptop was manufactured in 2014 and has a LCD. That was only 5 years ago. Fixed backlight, no light ambience sensor.
Cheap laptops today like the HP Pavilions of the world still use fixed-backlight LCDs as well.
If you're using dark mode to save battery on a laptop it's not working.