How to make date picker 'Ok'- and 'Cancel' button text black in Dart Flutter?

I have a date picker in Flutter. I want to make the 'Ok' and 'Cancel' button text black. But I can't find the correct theme setting.

Code displaying the date picker:

Future<void> selectDate(
      BuildContext context,
      DateTime initialDate,
      TextEditingController controller,
      Function(DateTime picked, TextEditingController controller) onDatePicked,
      String label) async {
    final DateTime picked = await showDatePicker(
        context: context,
        builder: (BuildContext context, Widget child) {
          return Theme(
            data: ThemeData(
              primarySwatch: Colors.grey,
              splashColor: Colors.black,
              textTheme: TextTheme(
                subtitle1: TextStyle(color: Colors.black),
                button: TextStyle(color: Colors.black),
              ),
              accentColor: Colors.black,
              colorScheme: ColorScheme.light(
                  primary: Colors.green[600],
                  primaryVariant: Colors.black,
                  secondaryVariant: Colors.black,
                  onSecondary: Colors.black,
                  onPrimary: Colors.white,
                  surface: Colors.black,
                  onSurface: Colors.black,
                  secondary: Colors.black),
              dialogBackgroundColor: Colors.white,
            ),
            child: child,
          );
        },
        initialDate: initialDate.toLocal(),
        firstDate: DateTime(2015, 8).toLocal(),
        lastDate: DateTime(2101).toLocal(),
        fieldLabelText: label);
    if (picked != null) onDatePicked(picked, controller);
  }

How it currently looks:

Thanks!