copyWith method
- BorderSide? side,
 - double? eccentricity,
 
override
    Returns a copy of this OutlinedBorder that draws its outline with the
specified side, if side is non-null.
Implementation
@override
CircleBorder copyWith({BorderSide? side, double? eccentricity}) {
  return CircleBorder(side: side ?? this.side, eccentricity: eccentricity ?? this.eccentricity);
}