borderSide property
final
Defines the color of the border when the button is enabled but not pressed, and the border outline's width and style in general.
If the border side's BorderSide.style is BorderStyle.none, then an outline is not drawn.
If null the default border's style is BorderStyle.solid, its BorderSide.width is 2.0, and its color is a light shade of grey.
Implementation
final BorderSide borderSide