* {
  box-sizing: border-box;
}

body {
  margin: 0;
  padding: 0;
}

header {
  background-color: pink;
  padding: 10px;
  text-align: center;
}

/* Style the topnav */
ul.topnav {
  display: flex;
  list-style-type: none;
  margin: 0;
  padding: 0;
  background-color: gray;
}

/* Style links in topnav */
ul.topnav li a {
  display: block;
  color: #f1f1f1;
  padding: 14px 16px;
  text-decoration: none;
}

/* Change color on hover */
ul.topnav li a:hover {
  background-color: #dddddd;
  color: black;
}

div.flex-container {
  display: flex;
  flex-direction: row;
}

div.flex-container > div {
  background-color: lightyellow;
  margin: 10px;
  outline: 10px solid lightgreen; /* looks like a colored margin */
  flex: 1;
}

/* Style the footer */
footer {
  background-color: gray;
  color: black;
  padding: 1px;
  text-align: center;
}

/* Use media query and show the flex items vertically if screen width is less than 600px */
@media screen and (max-width:600px) {
  div.flex-container {
    flex-direction: column;
  }
}