Support `prefers-color-scheme` CSS media query

[JammyDodger]

We’ve had the auto-dark feature for awhile now.

Automatic Dark Mode color scheme switching

You can now set up your Discourse site to automatically switch color schemes when the user’s device is in dark mode. For a quick preview, head over to the try.discourse.org instance and toggle your device’s dark mode on and off to see this new feature in action. (This feature not yet enabled on meta.) This brand new feature relies on changes to how themes and color schemes interact. If your site uses a custom theme or unofficial plugins, it is most likely not compatible with automati…