Reminds me of the meme of the person shoving a stick into their bikes wheel then complaining about ______. Used to get it a lot here with people complaining about sites being broken because they were hard blocking essentially all javascript, the issues are your own making here...

In my case firefox used the OS's (Windows 10) dark mode preference.

In about:config the layout.css.prefers-color-scheme.content-override setting was on 2 (system). Setting it to 3 (browser) turned the dark mode off.