If you look at the description in TFA, it sounds like this isn't a proper stylesheet, it's a heavy Invert Colors implemented specifically for Wikipedia.
If we're going to be indiscriminately inverting colors and trying to piece the page back together anyway, I strongly recommend using Dark Reader [0] instead and getting the benefits of this globally. It's open source and very good. I installed it when I first got an Android phone that supported dark mode and haven't even really noticed it since then, it's just as though every webpage in the world started respecting my phone's global dark mode toggle.
By far my favorite extension is Dark Reader - no need to rely on every website to implement a dark mode
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I think the OP link would have had the parameter included:
https://en.wikipedia.org/wiki/Wikipedia:Dark_mode_(gadget)?w...
from a quick glance it adds
to HTML style. What indeed works, but is a bit sloppy way to do a dark mode. Making usable colour palate and adjusting typography for inverted mode takes little more effort, sadly.