html {
  height: 100%;
}

body {
  min-height: 100%;
  margin: 0;
  padding: 2em;
  box-sizing: border-box;

  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;

  background-color: #839;
}

img {
  width: 90vmin;
}