Ah, I'm glad you mentioned that; I took a look at it earlier, but I think what would be most helpful is just the most minimal set of CSS we could put in curly braces after prefers-color-scheme: dark. In other words, a patch to https://news.ycombinator.com/news.css that would strictly only add lines—and preferably the most minimal that would do the trick.
I haven't inspected their quality or thoroughness myself, but it seemed like a certain amount of that was implied