<div class="container">
  <div class="box">
    BOX
  </div>
</div>

.box {
  margin-inline: auto;
}

/* その他のスタイル */

body {
  padding: 24px;
  font-family: sans-serif;
}

.container {
  border: 1px solid #ccc;
  padding: 24px;
}

.box {
  background-color: rgb(29, 155, 240);
  color: white;
  display: grid;
  font-size: 20px;
  font-weight: bold;
  height: 200px;
  place-content: center;
  width: 300px;
}