Add support for automatic dark mode (@media query `prefers-color-scheme`)

Gromit made the remark last year when the DarkMode extension was added that it would be great to adhere to a visitor's prefer-color-scheme setting.

Upstream documentation suggests:

In 2024, the recommend way for MediaWiki skins to use dark mode is to make use of CSS custom properties and define a dark mode palette that applies in the scenarios where the user has expressed a preference for dark mode at the operating system level (using prefers-color-scheme) or opted into dark mode via the UI.

After briefly discussing the topic with French archers, it seems that now is a good time to submit this issue to the archwiki team.