Firefox 67 also the first browser that ships with `prefers-color-scheme` (aka dark mode) after Safari. https://developer.mozilla.org/en-US/docs/Web/CSS/@media/pref... And it does the right thing and disables the feature when enabling the resist fingerprinting option. I did a small survey and only very few websites have developed a second dark theme: https://zimbatm.com/DarkMode
Dark mode, for the first time ever, has legitimate, objective reasons to want, now that we have pure LED displays: a black LED pixel consumes no energy.
Migraines aren't an uncommon problem.
I think that is a fairly objective reason driving the preference.