> Why can't the User Agent allow the user to set themes, with some sensible defaults provided?
I think you'll be happy to learn about prefers-color-scheme: https://developer.mozilla.org/en-US/docs/Web/CSS/@media/pref...
Chrome hasn't implemented it yet (Firefox and Safari did), but there's some activity here: https://bugs.chromium.org/p/chromium/issues/detail?id=889087
As far as I understand it, a website would have two stylesheets (dark and light) and your browser would pick which one should be used. Websites would still have two themes, but the activation process would get automated. This would allow the browsers to finally implement a "I prefer dark themes" setting.