<main>
  <div class="center-me">
    Block-level centered element
  </div>
</main>

body {
  background: #40BDB7;
}

main {
  background: white;
  margin: 20px 0;
  padding: 10px;
}

.center-me {
  margin: 0 auto;
  width: 200px;
  background: black;
  padding: 20px;
  color: white;
  border-radius: 5px;
}