Is it possible to keep resist fingerprint enabled and only add an exception for dark mode?
set privacy.fingerprintingProtection.overrides='+AllTargets,-CSSPrefersColorScheme',
set privacy.fingerprintingProtection='true', and
set privacy.resistFingerprinting='false'.
As far as I know, privacy.resistFingerprinting and privacy.fingerprintingProtection behave the same with these overrides (minus enforced light mode), but I haven't found much documentation on this, so please let me know if they don't.
It's all better explained at https://librewolf.net/docs/faq/#what-are-the-most-common-dow...
Having a look at about:config and searching 'resist' exposes a few other interesting options to customise like whitelisting etc.