After a lot of tests of other similar extensions, I find this one particularly well done, that's why i'm leaving this review. I love that it handles clicked links (different color once visited), contrarily to most of the other similar addons. I only found one doing this, and it's not open source, with less options. Here, you can choose several methods of darkening, and apply them to different part of the same website or domain (if the global setting breaks only some pages), by URL segments (domain, website, page, subpages, etc). I've not found this depth of settings in other addons. The customization of colors for different elements is easy and when some could say it's not as "fancy" as other addons, the interface is very clear and intuitive. And I also like that it is open source.
Developer, really well done. Your extension is the closest to perfection for what it does, imo.
Could you check if there is a way to darken the menu of firefox (left-click one, and when opening a folder of favorites in the favorites bar)? I'm afraid it's a limitation in Firefox Quantum, if you can't do anything about it, i totally understand. You have my 5 stars anyway :)

Edit: thanks for the reply and advice! I already have the dark themes enabled, I'll have to wait until Mozilla expand it to these areas of their browser.
Edit 2: oh, high contrast. Brutal, but indeed it works. Thank you for the tip! ;)