Librewolf turns on resist fingerprinting by default. If you go into about:config and disable resist fingerprinting, you can re enable light/dark mode preferences being recognized by the browser.
Or you could install the Dark Reader extension, but that will overwrite all CSS and approximate Dark Mode rather than use a customized CSS stylesheet.
Or you could install Vanilla Firefox and use that only to develop while you use Librewolf to privately browse, which is what I do.
If there isn't more convenient way, I'll just disable RFP when needed. Thanks!
Welcome to the official community for LibreWolf.
LibreWolf is designed to increase protection against tracking and fingerprinting techniques, while also including a few security improvements. LibreWolf also aims to remove all the telemetry, data collection and annoyances, as well as disabling anti-freedom features like DRM. If you have any question please visit our FAQ first: https://librewolf.net/docs/faq/
To learn more or to download the browser visit the website: https://librewolf.net/
If you want to contribute head over to our Codeberg: https://codeberg.org/librewolf