"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.