When you click on a link to the news.ycombinator.com domain, what should happen if you prefer dark mode? And vice versa?
In comparison, the pure-CSS ideas that some people are posting would work automatically if your browser supports dark mode, and would enlarge the (cached with long ttl) CSS file only very slightly.
Unfortunately, there are some of us who also like the solarized type light mode in HN more than dark mode, but we use dark mode at the system level because the OS light mode is awful.
Pure CSS would be quite cool, but we'd probably also need a snippet of JS to add a button allowing to switch between light and dark mode. (There's probably a way to do it without JS that someone will hopefully comment too.)
In comparison, the pure-CSS ideas that some people are posting would work automatically if your browser supports dark mode, and would enlarge the (cached with long ttl) CSS file only very slightly.