Automatic dark mode based on prefers-color-scheme preference

Once dark mode is ready for widespread use, we can enable it (for guest/new users at least) based on prefers-color-scheme: dark media query

Possibly also add an option to User Settings > Preferences for UI, so you have Light, Dark, and Automatic (from system UI)

Good idea of --color-mode var in https://www.smashingmagazine.com/2024/03/setting-persisting-color-scheme-preferences-css-javascript/