If you mean that extension, that doesnt work. It downloads a separate firefox binary and creates a separate profile for each app with customized CSS.
It is very nice and I will try to recreate it using Librewolf Flatpak, but I wouldnt (and cant) use that app currently, as my system hard hardened_malloc as memory allocator which breaks Firefox. Either compile it with a custom parameter or use the flatpak and remove that env variable.