header {
  text-align: center;
  background: #007c01;
  width: 100%;
  position: fixed;
  top: 0;
  left: 0;
  z-index: 999;
}
header a {
  display: inline-block;
  font-size: 2.3rem;
  font-weight: 700;
  padding: 2rem;
  color: #fff;
}

main {
  margin-top: 6.3rem;
  padding: 3rem 2rem;
}

.source {
  font-size: 1.5rem;
  color: #aaa;
  font-style: italic;
  line-height: 1.4;
}