Thanks to Victoria Drake, I discovered the color-scheme #CSS declaration and added it to my site. It's still a draft, but it's a way to tell browsers what themes your site has available. If the user has set a preference that would trigger the @prefers-color-scheme media query, browsers that implement it should adjust their chrome to match that theme.
I already have Firefox set to dark mode and have extensions customizing other theming, so I don't know if it affected anything on my site. But it's one line and its good to future proof and give more priority to user choice.
I'm working on a #design update of my site based on new.css. It's available as an alternate stylesheet, so if you want to check it out and you're using a browser that supports it, you can apply the New.css Styles stylesheet. When I'm happy with where it's at, I'll probably just switch the two stylesheets, so the current one sill still be available as an alternate.
“Higher Level Custom Properties” are Custom Properties that control multiple other CSS Properties. Although still a proposal it's worth having a look already as they're really exciting.
https://www.bram.us/2020/12/30/the-future-of-css-higher-level-custom-properties-to-control-multiple-declarations/
An in-depth article on how to handle short and long content in CSS.
https://ishadeed.com/article/css-short-long-content/