: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, With this change, it should be extremely simple to maintain both dark and light modes in parity.