.wrapper {
  display: flex;
  min-height: 100vh;
  flex-direction: column;

  header, footer {
    background: #82caff;
    flex: 0 0 6em;
  }
}

.body {
  display: flex;
  flex: 1;
  flex-direction: column;

  main {
    flex: 1;
    background: #fecccc;
  }
  aside, nav {
    flex: 0 0 12em;
    background: #c9c;
  }
  nav {
    order: -1;
  }
}

@media (min-width: 768px) {
  .body {
    flex-direction: row;
  }
}