:root {
  --topnav_height: 40px; /* Height of the top navigation bar */
  --body_height: calc(100% - var(--topnav_height)); /* Height available to the webpage body */
  --centerpiece_width: calc(100% - 2.0 * var(--shelf_width));
}

.topnav {
  overflow: hidden;
  background-color: #333;
  height: var(--topnav_height);
  //position: fixed;
  //top: 0;
  //width: 100%;
}

.topnav a {
  float: left;
  color: #f2f2f2;
  text-align: center;
  padding: 7px 16px;
  text-decoration: none;
  font-size: 17px;
  height: 100%;
}

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

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