leadingCellTextStyle property
Specify the date picker leading year, decade or century cell text style
Widget build(BuildContext context) {
 return Scaffold(
   appBar: AppBar(),
     body: Center(
       child: SfTheme(
         data: SfThemeData(
           dateRangePickerThemeData: SfDateRangePickerThemeData(
             leadingCellTextStyle: TextStyle(color: Colors.red,
               backgroundColor: Colors.green)
           )
         ),
         child: SfDateRangePicker(),
       ),
     )
  );
}
Implementation
final TextStyle? leadingCellTextStyle;