Match Sublime colors

- Add some of the changes introduced by [Sumolari/spacegray-eighties-ui](/Sumolari/spacegray-eighties-ui) fork, plus some of my own. This should be a closer match to the sublime theme.

- Refactored the variables file to reflect the same naming that uses [Base16 color scheme](http://chriskempson.github.io/base16/#eighties).