@media screen and (max-width: 700px) {
  .page {
    flex-direction: column;
  }
}
@media screen and (max-width: 400px) {
  #main a {
    float: none;
    width: 100%;
  }
}

body {
  background-color: black;
  font-family: sans-serif;
  margin: 0;
  display: flex;
  flex-direction: column;
}
a {
  color: #4da6ff;
}
blockquote {
  border-left: 5px solid #222;
  padding-left: 10px;
}

.header {
  padding: 10px;
  text-align: center;
  background: #1a406b;
  color: lightgray;
}
.header h1 {
  font-size: 40px;
}

#main {
  overflow: hidden;
  background-color: #222;
}
#main a {
  float: left;
  display: block;
  color: lightgray;
  text-align: center;
  padding: 14px 20px;
  text-decoration: none;
}
#main a:hover {
  background-color: #444;
  color: white;
}

* {
  box-sizing: border-box;
}
.page {
  flex: 1;
  display: flex;
  flex-wrap: wrap;
}
.side {
  width: 20%;
  background-color: #111;
  color: lightgray;
  padding: 20px;
}
.main {
  width: 80%;
  background-color: black;
  color: lightgray;
  padding: 20px;
}
.footer {
  padding: 20px;
  text-align: center;
  background: #ddd;
}