:lipstick: Abstract tones (base0, etc.) into syntax variables

Solarized creator Ethan Schoonover [explains] 

| Solarized flips between light and dark modes. In each mode, four monotones 
| form the core values (with an optional fifth for emphasized content).
| …
| This makes colorscheme inversion trivial. 

Switching between dark and light modes should be a matter of redefining a handful of base tones, ideally in a single stylesheet. 

I've abstracted all direct references to the base tones into syntax variables, making it extremely simple to maintain both dark and light modes in parity. 

[explains]: http://ethanschoonover.com/solarized#usage-development