thumbStrokeWidth property

Specifies the stroke width for the thumb in the SfSlider, SfRangeSlider, and SfRangeSelector.

Defaults to null.

This snippet shows how to set thumb stroke width in SfRangeSliderThemeData.

SfRangeValues _values = SfRangeValues(4.0, 7.0);

Scaffold(
  body: Center(
      child: SfRangeSliderTheme(
          data: SfRangeSliderThemeData(
              thumbStrokeWidth: 2,
              thumbStrokeColor: Colors.red,
          ),
          child:  SfRangeSlider(
              min: 2.0,
              max: 10.0,
              values: _values,
              onChanged: (SfRangeValues newValues){
                  setState(() {
                      _values = newValues;
                  });
              },
          )
      ),
   )
)