The default colors defined in smtk::attribute::Definition presuppose the
use of a dark font. This method tests the font lightness and, if the font
color is light, it adapts the input color to contrast with it.

TODO: We may not want to define color in smtk::core. Instead, we may want
      to use Qt's convention of naming entities within a color palette
      rather than hard-coding colors; these descriptions should also live
      in the smtk::extensions::qt library, where they are used.