You can force Firefox to display dark mode in web content (even with privacy tweaks enabled to resist fingerprinting or tracking), by setting the two following hidden prefs in your user.js:
// PREF: enable a Dark theme for browser and webpage content
// [TEST] https://9to5mac.com/
user_pref("ui.systemUsesDarkTheme", 1); // HIDDEN
user_pref("browser.in-content.dark-mode", true); // HIDDEN