* {box-sizing: border-box;}

body {
  margin: 0;
}

.header {
  overflow: hidden;
  background-color: #f1f1f1;
  padding: 20px 10px;
}

a {
  float: left;
  color: black;
  text-align: center;
  padding: 12px;
  text-decoration: none;
  font-size: 18px;
  line-height: 25px;
  border-radius: 4px;
}

a.text {
  font-size: 25px;
  font-weight: bold;
}

a:hover {
  background-color: #ddd;
  color: black;
}

a.active {
  background-color: dodgerblue;
  color: white;
}

.header-right {
  float: right;
}

@media screen and (max-width: 500px) {
  .header a {
    float: none;
    display: block;
    text-align: left;
  }

  .header-right {
    float: none;
  }
}

.column {
  float: left;
  width: 50%;
  padding: 10px;
  height: calc(100vh - 89px)
}

.row:after {
  content: "";
  display: table;
  clear: both;
}

.row {
  margin-left: 15px;
  margin-right: 15px;
}

@media screen and (max-width: 600px) {
  .column {
    width: 100%;
    height: calc((100vh - 89px)*0.5)
  }
}