"prefers-color-scheme always says light mode."
Currently, when privacy.resistFingerprinting is enabled, it's not possible to set our preference to dark mode, under any circumstance: not through system settings, not through browser settings, not in inspect mode, and it doesn't appear it can even be overridden with add-ons.
I believe the Firefox devs have already ruled in on this request, and they won't allow this preference to be changed when privacy.resistFingerprinting is enabled.
However, it occurred to me, that a better solution would simply be to change the default anyway, to dark mode, for a number of reasons:
It seems setting the default to dark wouldn't just make for a better, more preferable experience, it might actually be better for privacy and obfuscation overall.
Thanks for submitting an idea to the Mozilla Connect community! Your idea is now open to votes (aka kudos) and comments.