The highlight color of the button's InkWell.
The highlight indicates that the button is actively being pressed. It appears on top of the button's child and quickly spreads to fill the button, and then fades out.
If textTheme is ButtonTextTheme.primary, the default highlight color is transparent (in other words the highlight doesn't appear). Otherwise it's the current theme's highlight color, ThemeData.highlightColor.
final Color? highlightColor;