body {
  margin: 3.5rem 0 0 0;
}

.menu {
  position: fixed;
  left: 0;
  top: 0;
  right: 0;
  max-height: 100%;
  z-index: 10;
}

@media (min-width: 1200px) {
  body {
    margin: 0 0 0 17rem;
  }

  .menu {
    left: 0;
    top: 0;
    bottom: 0;
    width: 17rem;
  }
}