fxcss uninstall

fxcss uninstall
fxcss uninstall --profile default-release

Removes the managed theme and its user.js preference block. Recorded files are removed only when their hashes still match; added, edited, unreadable or unverifiable files are kept. If nothing needs to be retained, the original chrome/ backup is restored. Otherwise the backup stays alongside the retained files so it cannot overwrite your work.

If the profile originally had no chrome/, uninstalling after an upgrade returns it to that state when no files need to be kept. Restart Firefox to see the result.