Match Sublime colors

This PR uses some of the changes introduced by [Sumolari/spacegray-eighties-ui](/Sumolari/spacegray-eighties-ui) fork which are a closer match to the sublime theme.

I've added a few more tweaks and refactored the variables file to reflect the same naming that uses [Base16 color scheme](http://chriskempson.github.io/base16/#eighties).