Change the definition of a color-pair.
pair_number
The number of the color-pair to be changed (1 - (COLOR_PAIRS-1)).
fg
Foreground color number (-1 - (COLORS-1)).
bg
Background color number (-1 - (COLORS-1)).
If the color-pair was previously initialized, the screen is refreshed
and all occurrences of that color-pair are changed to the new
definition.