Rolled light and dark themes into one file

- The highlighting function now expects an array for its color
  arguments. The first value in the array is the color to use for the
  dark version of the theme, the second number is light.
- Everything has been set up to switch correctly on set bg=light/dark.
  The theme queries &background and makes color choices appropriately.
  Vim will re-run coloring when the value of &background changes.
- The light theme file has been removed.