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.