Check if you have resistfingerprinting config turned on in about:config
. It being on picks the light color scheme by default. Mull has this privacy feature on by default and while the extension detects there is a dark color scheme, the config forces light (bright) color scheme.
Oh yes, the open source enthusiast that does removed all but seek out the most competitive softwares for free and adds nothing but constant moaning and complaining.
You’re doing open-source a great service.