<div class="container">
    <div class="centered-element">center</div>
  </div>

.container {
  width: 100vw;
  height: 100vh;
  display: flex;
}

.centered-element {
  width: 100px;
  height: 100px;
  background-color: blue;
  color: white;
  text-align: center;
  line-height: 100px;
  margin: auto;
}