Consider support for prefers-color-scheme media query (light / dark mode support)

[KZeni]

Ref: https://twitter.com/Keithamus/status/1055179677277384705

Safari Tech Preview now supports a prefers-color-scheme: dark CSS media query that is used when macOS Mojave is in dark mode.

I would love to see this plugin support this separate media query by offering both a "Regular/Normal/Default" (what the plugin currently offers) and "Dark" set of colors to be used (Dark then being optional, of course, and simply offering the same amount of scheme customization as the regular color set.)

Correct me if I'm wrong, but it could be as simple as duplicating the existing admin color controls to have a separate "Dark" set (possibly include a new box or have it as a new column in the existing box... including how it's used by prefers-color-scheme with browsers, of course) and then setting up the dark color customizations to output just like the existing setup with them wrapped in the prefers-color-scheme: dark media query.