stylus extension let you write styles per sites. there's also a centralized place to find already written styles. I use https://userstyles.org/styles/161954/hackernews-one-dark

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 also added a screenshot if that is easier - https://imgur.com/a/mxaNky4

View source, add a style tag to the header after the official styles, select the tag and use f2 to edit as text, paste the styles in.

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 :)

You could add it to your user stylesheet?

Where is that?