<div class="d-flex flex-column justify-content-center align-items-center">
<h1 class="display-1">Display 1</h1>
<h1 class="display-2">Display 2</h1>
<h1 class="display-3">Display 3</h1>
<h1 class="display-4">Display 4</h1>
<div>

html, body {
  display: grid;
  height: 100%;
}