Old post but it looks like its there: (Warning, Bad Place ahead)
https://www.reddit.com/r/LibreWolf/comments/u0hx0t/customization_with_css/
LibreWolf
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
Yup I've been using Firefox-UI-Fix for a long time now.
Used it back on Firefox and it worked fine when I switched to Librewolf.
Yes. Firefox has yet to kill userChrome.css and the Librewolf devs didn't disable it.