I tweaked and combined two existing Stylus entries called "Hacker News Readable" and "Hacker News Readable Dark" to make a layout that changes from light to dark mode with the system settings. https://jsfiddle.net/DHepworth/e4kwpr9j/2/
I just right clicked a few pages -> Save page as (static HTML/CSS/etc) then hack on the end of the news.css file. You can just append my css to that and you'll see exactly what I saw :)