OK, I tried many dark readers / dark mode add-ons, and I have this one for few months now (maybe 6 months), and I should say that's the only one with interesting options at one click away (for example: you can de-activate the dark mode on the fly for a given website and keep the dark mode everywhere else)

It has a nice default dark mode out of the box (although you can custom your dark mode, but i reset mine to default as it is well thought)

Thanks a lot developer, very nice job!