The irony is that light mode is objectively the correct way to use things.
Of your criteria is battery life, dark mode is most likely better
If your criteria is eye damage/strain, then IIRC the research is divided on this topic
black on white is easier to read than white on black full stop, no astigmatism necessary.
https://esa.org/communication-engagement/2018/08/03/resource...
ambient lightning is highly recommended to not strain your vision.
yes it's all preference, vision is subjective, but being surprised that dark mode isn't best is in this context... weird.