.sidebar {
    margin: 0;
    padding: 0;
    width: 73px;
    background-color: #f1f1f1;
    position: absolute;
    overflow: auto;
}

.sidebar a {
  display: block;
  color: black;
  padding: 16px;
  text-decoration: none;
}

.sidebar a.active {
  background-color: #4CAF50;
  color: white;
}

.sidebar a:hover:not(.active) {
  background-color: #555;
  color: white;
}

div.content {
  margin-left: 73px;
}

@media screen and (max-width: 700px) {
  .sidebar {
    width: 100%;
    height: auto;
    position: relative;
  }
  .sidebar a {float: left;}
  div.content {margin-left: 0;}
}

@media screen and (max-width: 400px) {
  .sidebar a {
    text-align: center;
    float: none;
  }
}