Resolves #1091, by automatically making the site’s background-color dark, if the user prefers a dark color scheme.

The dark background color is technically a very dark brown. I think it makes sense to not make it pitch black (like #000), so that the borders of the remote screen are still discernible even if it happens to render all black. (E.g., the “no signal” placeholder.)

Two notes:

• For the onscreen keyboard, it’s currently tricky to make it dark, because its color values are mostly hard-coded. We’d have to refactor that first, but I’m not sure how worthwhile that is. For now, I think it’s bearable.
• The dialogs must stay bright, because otherwise they wouldn’t have enough contrast against the dark overlay background.