<main>
  <div>
    This is a block-element which is centered and stacked on top of another block-element.
  </div>
  <div>
    This is a block-element which is centered and stacked on top of another block-element. I am larger than the other blocks because of the CSS selector :nth-child()
  </div>
  <div>
    This is a block-element which is centered and stacked on top of another block-element.
  </div>
</main>

body {
  background: #40BDB7;
  font-size: 80%;
}

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

main div {
  background: black;
  margin: 0 auto;
  color: white;
  padding: 15px;
  margin: 5px auto;
  border-radius: 5px;
}

main div:nth-child(1) {
  width: 200px;
}
main div:nth-child(2) {
  width: 400px;
}
main div:nth-child(3) {
  width: 125px;
}