html, body {
  height: 100%;
  width: 100%;
}

body {
  padding: 0;
  display: flex;
  justify-content: center;
  margin:0;
}

.root {
   width: 768px; /* typical tablet width */
  max-width: 100%; /* shrink on smaller screens */
  min-height: 100%; /* full height if you want */
}