The best on the market imo.

Gradients on page previews are inverted

great addon. it makes wikipedia way more comfortable to read.
only problem is that some elements are displayed black on grey. i.e. math formulas.
could probably be solved easily by adding a lighter background to everything with src containing "math/render/svg/".
otherwise it's perfect imo.

duckduckgo's first result for "wikipedia dark mode firefox" did not disappoint