Abstract comment tones

Abstracting `.editor .comment` to `syntax-comment-color` was a no-brainer.
There may be more appropriate ways to color `.entity &.attribute-name` and
`.variable &.constant`, but maybe we'll see how it shakes out when we invert
to light colors.
Abstract emphasized tones
Remove direct references to base tones from base
Remove base tone references from syntax styles
Restore base tones references to terminal syntax
Merge pull request #26 from clhynfield/abstract-all-tones

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