I used "Override colors" under Settings -> Accessibility to set nice dark colors. Then I used userChrome.js from https://github.com/AlexVallat/firefox-scripts/

I used "Override colors" under Settings -> Accessibility to set nice dark colors. Then I used userChrome.js from https://github.com/AlexVallat/firefox-scripts/ https://github.com/AlexVallat/firefox-scripts/ to add a button which toggles the browser preferences "browser.display.use_system_colors" and "browser.display.document_color_use", so I can still access bright mode if needed.