Adds dark theme

This pull request adds a dark theme to the themes directory. I felt I had to create it because I personally use a custom Terminal (Mac), which has a partially opaque background and a lighter background color. It becomes a little bit harder to see lighter colors, and I figured somebody else could benefit from this theme.

Nice app! :+1: heres a screenshot of how the theme looks on my Mac.

![screenshot](http://upimg.me/01441947c956839cdb6ad423272ddb31.png)