The color and width of the checkbox's border.
This property can be a MaterialStateBorderSide that can specify different border color and widths depending on the checkbox's state.
Resolves in the following states:
If this property is not a MaterialStateBorderSide and it is non-null, then it is only rendered when the checkbox's value is false. The difference in interpretation is for backwards compatibility.
If this property is null, then CheckboxThemeData.side of ThemeData.checkboxTheme is used. If that is also null, then the side will be width 2.
final BorderSide? side;