Sets the current background color, using the palette.
procedure SetBkColor(ColorNum: Word);
Background colors range from 0 to 15, depending on the current graphics driver and current graphics mode. On a CGA, SetBkColor sets the flood overscan color.
SetBkColor(N) makes the Nth color in the palette the new background color. The only exception is SetBkColor(0), which always sets the background color to black.
Must be in graphics mode.