[emaro]

You can in fact add dark mode with a single line of code (CSS, not JS), as long as you don't change any colors:

    :root { color-scheme: light dark; }

I don't think you need a library for that.

[rollcat]

You don't need CSS either!

    <meta name="color-scheme" content="light dark" />

[OskarS]

Not a web dev, but I saw this on a stream at some point and found it delightful. Surprisingly effective!

    html { filter: invert(100%); }

[TazeTSchnitzel]

Though you probably want to apply the same filter again to non-UI images so they won't look weird.