html, body {
  margin: 0;
  height: 100%;
}

#container {
  height: 100%;

  display: flex;
  flex-direction: column;
}

header {
  text-align: center;
}

header > h1 {
  margin: 0;
}

main {
  flex: 1;
}

footer {
  text-align: center;
}