"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 Very long manual probably outdated.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, should be removed.
Is obviously no longer being developed further.
Latest comments on their website are from 2014.

mantap

Es arbeitet nicht.

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