Add support for Dark Mode.

[dmitshur]

This is a beginning...

A prototype is available at https://dmitri.shuralyov.com/projects/home-dark/.

References:

• https://drafts.csswg.org/mediaqueries-5/#prefers-color-scheme
• https://paulmillr.com/posts/using-dark-mode-in-css/ (/cc @paulmillr)