borderColor property

Specifies the knob border color.

Defaults to null.

Widget build(BuildContext context) {
   return Container(
       child: SfRadialGauge(
         axes:<RadialAxis>[RadialAxis
         ( pointers: <Pointer>[
            NeedlePointer(
             knobStyle: KnobStyle(borderColor: Colors.red,
             borderWidth: 2),
          )])]
       ));
}