selectionTextStyle property
Specify the date picker selected cell text style for the single and multiple selection and also for the start and end range of single and multi-range selection.
Widget build(BuildContext context) {
 return Scaffold(
   appBar: AppBar(),
     body: Center(
       child: SfTheme(
         data: SfThemeData(
           dateRangePickerThemeData: SfDateRangePickerThemeData(
             selectionTextStyle: TextStyle(decoration:
               TextDecoration.overline)
           )
         ),
         child: SfDateRangePicker(),
       ),
     )
  );
}
Implementation
final TextStyle? selectionTextStyle;