.center {
    display: block;
    margin-left: auto;
    margin-right: auto;
    width: 100%;
    max-width: 500px;
  }

.center-a {
  margin-left: auto;
  margin-right: auto;
  width: 75%;
  display: flex;
    flex-direction: column;
    justify-content: center;
}

h1 {text-align: center}