To be clear: I only propose that users can explicitly opt-in via creating a, by default hidden, preference to willfully disclose a chosen preferred color scheme and still be able to use Firefox's general fingerprinting resistance. (
ui.systemUsesDarkTheme offers itself since it is already used in that context and is hidden)
One could also or instead overload
ui.systemUsesDarkTheme with another, entirely new integer value that explicitly says: Use the metric decided in bug 1529323, regardless of
privacy.resistFingerprinting already has various papercuts that prevent some users that would like to at least partially benefit from it from using it (e.g. the default 1000x1000px(?) window size on restore), I'd just like to avoid adding even more.