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
It’s certainly existed for years and it’s a nice release, such as it is. However, the absolute lack of IPv6 and abject refusal to incorporate WireGuard for ReasonsTM led to me going back to OpenBSD and OpenWRT, personally. They seem to have a WIP towards finally incorporating WireGuard, but IPv6 is still AWOL, which is fatal for my use-case.