Fix theme "background-mode" variable definition

Set the theme background-mode variable to the string "light" or "dark" rather
than the equivalent symbol. This fixes the "Symbol's value as variable is
void: dark" (or light) error that prevents the *Custom Theme* buffer loading
when trying to customize a Solarized theme.
Add global color variables and related functions

It's sometimes useful to access the Solarized colors from outside the
definitions for the specific color-theme or custom theme. Add some features
related to this, including:

- Make color values accessible as `solarized-base01`, `solarized-blue`, etc.

- Add `solarized-theme-hook` which runs when the color definitions are changed,
  allowing contextual access to the base color variables.