Dark mode based on the Apollo Midnight theme

[hwillson]

It would be awesome to wire up full dark mode support, leveraging elements from Apollo's Midnight theme.

@daniman @jglovier - this won't make it into 3.0, but I'm adding it here so we don't lose sight of it. Thanks for the suggestion!

[jerelmiller]

With the update to Apollo's new Orbit theme in #1180, dark and light modes are now supported using this theme. As such, I'm going to go ahead and close this as completed 🙂