trailingDatesTextStyle property
Specifies the text style for the trailing month cell dates.
Widget build(BuildContext context) {
 return Scaffold(
   appBar: AppBar(),
     body: Center(
       child: SfTheme(
         data: SfThemeData(
           calendarThemeData: SfCalendarThemeData(
             trailingDatesTextStyle: TextStyle(decoration:
               TextDecoration.lineThrough,color: Colors.amber)
           )
         ),
         child: SfCalendar(),
         ),
     )
  );
}
Implementation
final TextStyle? trailingDatesTextStyle;