<!--  our html has a container which contains a div we wish to center-->
<div class="container center">
  <div class="inner">
  
  </div>
</div>
    

html,
body {
  height: 100%;
}

.container {
  background: black;
  width: 100%;
  height: 100%;
  font-size: 2rem;
}

.inner {
  background: turquoise;
  width: 50px;
  height: 50px;
  border-radius: 50%;
}

/* This is the main CSS that centers the div */
.inner{
   margin: auto;
}