Amazing! It offers a variety of dark themes, but even the default one "Dark mode #1 (simple dark)" is so good that I didn't really try the others. There is thought put to the color palette, it's not just pitch black but a lighter dark much more pleasing to the eye. Furthermore, the turn-off button is conveniently/easily accessible as there are some unavoidable cases when sites will be broken or incorrectly displayed.

However, I do have some concerns about privacy: why does it need permission to all our website data? Does it have display-customizations for certain sites so it needs to check at runtime which website is loaded?
If yes, ironically, the addons.mozilla.org is not affected by the dark mode :P