Comment by TheWoodsy

Dark mode will be remembered if cookies are remembered. Resist blocks the website from knowing if your OS is light/dark so auto-dark/light, will be disabled unless resist is disabled.

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.