"It could be useful, but unfortunately, it doesn't work (at least not, with FF v.139.0)"

Not working at all. Very long manual probably outdated.

Quantum以降のFirefoxには対応していないようで、紹介されている説明の殆どの機能は削られています。

After you instal it (right click the add button, save to file, then instal from file) and click on options, there is a message that it's mostly non functional and asks you to complete a survey about what features you want etc as they're considering whether to update or discontinue it.
I suggested they put that in the addon download page!

Theres only 2 buttons: background colour and text colour I didn't try them.

Does not work and cannot resize fonts as admitted by authors.

Doesn't work. Waste of time

mantap

Es arbeitet nicht.

hmm so.. it doesn't change font!!