v0.20.3

Added

• The theme skill writes a theme's syntax and pair. It ports a palette's
  code colors into the optional syntax section, which colors the code in
  lich native's review, diff and file views, and pairs the light and dark
  variants of a palette so the native window follows the desktop between them.
  validate.mjs checks both the way lich does: an unknown highlighting tag or
  modifier, a style with no color and no font style, a theme that is its own
  pair, or a pair of its own scheme.