<div class="container center">
  <span class="demo">Thing that should be centered on the page</span>
</div>

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

.center {
  display: flex;
  place-content: center;
}

.demo {
  background-color: gray;
  color: white;
  padding: 0.25em;
}