Support `prefers-color-scheme` CSS media query

[pmusaraj (Penar Musaraj)]

There is an experimental theme component that does the automatic switch based on media queries:

Dark Mode Automatic Theme Switcher

Discourse now supports Automatic Dark Mode color scheme switching in core, this component is now deprecated and should no longer be used. Repository: https://github.com/pmusaraj/discourse-dark-mode Installation instructions: How do I install a Theme or Theme Component? What this does? This component allows automatic switching to a dark theme when the browser is set to dark mode. This works for logged-in users only on all browsers that support the prefers-color-scheme media query…

Because window.matchMedia is not available server-side, as you note, the component evaluates in JS whether prefers-color-scheme matches the current theme, and if not, sets the correct theme and reloads the site with the updated theme.