Support dark/light mode based on browser's preferences

[humitos]

We should start talking about how to support this and make it nice at least for the most common use cases.

Reference:

• Allow disable of calculateStableLatestVersionWarning #298

[pradyunsg]

I'd also appreciate if themes could provide indicators to the JS client about this, since Furo maintains its own state for these details.

[humitos]

@pradyunsg would #197 and #282 would be enough for Furo? I'd like to build something where theme authors can configure easily without depending on us implementing the styles to match their themes, if possible.

[pradyunsg]

It would, yes! :)