Add a dark theme

[briandominick]

I cannot believe I'm only just now adding this, but this theme absolutely should have a version with light text on a dark background. This is the style I prefer personally, when done right. Admittedly, I have never done it, as no customer has asked me for it, but I would love to have all my docs in a site like Jekyll's docs or even GitHub's dark theme.

Also wondering if we should stop using "skins" as a way to describe the "look and feel" of the very front-end. It doesn't feel great to use terms like "dark" and "light" with that word in this context.

This should obviously wait until after #24, but I think it is a goal for relatively early (0.3.0?).