CSS: center a div (flexbox)

Use a parent container with flexbox alignment:

<div class="parent">
  <div class="child">Centered</div>
</div>

.parent {
  min-height: 60vh;
  display: flex;
  align-items: center;
  justify-content: center;
}

.child {
  padding: 1rem 1.25rem;
  border-radius: 12px;
  background: #111827;
  color: white;
}