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.

Doesn't work, should be removed.
Is obviously no longer being developed further.
Latest comments on their website are from 2014.

Doesn't work. Waste of time

Es arbeitet nicht.

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

"Because of the new extension system on Firefox, most of the functionality of the Theme Font & Size Changer is not available any more."

Fake extension, you can't change the font type or size, DO NOT INSTALL IT

Does not let me change font size.

It doesn't do what it says it does, it should be removed.

doesnt let you change font/font size anymore :(

This extension should be marked CLEARLY that it doesn't work anymore (or better - totaly removed). Don't waste your time attempting to install it - the only thing you will get is the information that it doesn't work in new FF versions.

使えない
（フォントはともかくサイズを変更したかったが色しか変更できなかった）