If you are illegally camped in a dark field and your phone, tablet or laptop is the only source of light, bright white images emit much more light that can give away your position than dark ones.
IIRC, the two primary things we did to conserve battery power were 1. Avoid certain games and 2. Turn down screen brightness.
In ordinary usage, the screen on our tablets was typically the biggest drain on the battery. This was especially true for tablets with a large screen.
I don't have (anec)data specific to dark mode on websites and power usage. I just know that using our devices carefully was the difference being kept occupied for three or four hours until we could sleep or running out after an hour and having nothing to do but talk, which is not a stealthy activity.
