This was a remnant from having separate solarized-dark and -light themes. We
only ever produce one theme now, so use the usual theme definition mechanism.
- Do a better job of setting up the right set of colors for various
configurations.
- Make the theme much more compact by not making duplicate face-spec elements –
e.g., only generate 256-color-specific elements if we don’t use the 16-color
color names for them, and only specify the background characteristic when the
face differs between dark and light.
- Add functions to toggle between dark and light modes (for any theme).
There is now a single theme that will use light or dark versions based
on the background-mode of the frame, this allows a mix of light and dark frames.
