highlightColor property Null safety
The secondary color of the button when the button is in the down (pressed) state. The highlight color is represented as a solid color that is overlaid over the button color (if any). If the highlight color has transparency, the button color will show through. The highlight fades in quickly as the button is held down.
Defaults to the Theme's highlight color, ThemeData.highlightColor.
final Color? highlightColor;