body {
  align-items: center;
  justify-content: center;
  background: green;
  color: white;
  display: flex;
  flex-direction: column;
  font-family: sans-serif;
  font-size: 2em;
  height: 100vh;
  margin: 0;
  overflow: hidden;
  padding: 0;
  width: 100vw;
  transition: background 0.25s ease-in;
}

body.flat {
  background: blue;
}

body.sharp {
  background: red;
}

h1,
h2 {
  margin: 0;
}

h1 {
  font-size: 6em;
}

h2 {
  font-size: 3em;
}