그다음에는 CalendarView에서 markedSelectedDate라는 새로운 객체를 만들어 현재 날짜에 selected: true를 설정하세요.

components/CalendarView.js

import React from 'react';
import {Calendar} from 'react-native-calendars';
import {StyleSheet} from 'react-native';

function CalendarView({markedDates, selectedDate, onSelectDate}) {
  const markedSelectedDate = {
    ...markedDates,
    [selectedDate]: {
      selected: true,
      marked: markedDates[selectedDate]?.marked,
    },
  };

  return (
    <Calendar
      style={styles.calendar}
      markedDates={markedSelectedDate}
      theme={{
        selectedDayBackgroundColor: '#009688',
        arrowColor: '#009688',
        dotColor: '#009688',
        todayTextColor: '#009688',
      }}
    />
  );
}

(...)