style.css

* {
  box-sizing: border-box;
}
body {
  display: flex;
  justify-content: center;
  height: 100vh;
  background-color: #f2f4f6;
  font-family: '맑은 고딕', sans-serif;
}

.calendar {
  border-radius: 10px;
  position: relative;
  width: 300px;
  height: 300px;
}

.calendar-box {
  display: block;
  position: absolute;
  top: 35px;
  left: 0;
  box-shadow: 5px 5px 10px rgba(0, 0, 0, 0.2);
  border-radius: 30px;
}
.calendar-box.hide {
  display: none;
}

h1 {
  margin: 20px 0;
  font-size: 43px;
  color: #af5b4a;
}
.calendar .input-area input[type='text'] {
  width: 200px;
  height: 35px;
  border: 2px solid #1e457f;
  padding: 5px;
}

/*-------.calendar-nav------*/
.calendar-nav {
  display: flex;
  margin-top: 5px;
  justify-content: space-between;
  padding: 20px;
  background-color: #5cd17b;
  border-radius: 20px 20px 0 0;
  text-align: center;
}
.calendar-nav span {
  display: block;
  font-size: 16px;
  color: #fff;
}
.calendar-nav span.nav-month {
  margin-bottom: 5px;
  font-size: 20px;
  font-weight: bold;
}

/*-------.calendar-grid------*/
.calendar-grid {
  padding: 15px;
  background-color: #fff;
  border-radius: 0 0 20px 20px;
}
.calendar-grid ul {
  display: grid;
  grid-template-columns: repeat(7, 1fr);
  text-align: center;
}
.calendar-grid ul li {
  padding: 15px 10px;
  cursor: pointer;
}

.calendar-grid ul li.today {
  border: 1px solid #5cd17b;
  border-radius: 50px;
  font-weight: bold;
}
.calendar-grid ul li:nth-child(7n + 1) {
  color: #ff4b3e;
}

.notCurrDay {
  color: #cecece !important;
}
.select {
  background: #5cd17b;
  color: #fff;
  border-radius: 50px;
  font-weight: bold;
}