.center_text { text-align:center; }

.page_header 
  {
  background-color: #333;
  color: white;
  padding: 15px 20px;
  position: fixed;      /* Keeps the header at the top */
  top: 0;
  left: 0;
  right: 0;
  z-index: 1000;        /* Keeps it above other elements */
  }

  .nav_bar { text-align:center; }

  .nav_bar a
  {
      color:white;
      margin-left:3vw;
      margin-right:3vw;
      text-decoration:none;
  }

.display_code
{
    display: block;
    background-color:powderblue;
    border: 1px solid #ddd;
    border-radius: 4px;
    padding: 8px;
    font-family: monospace;
    white-space: pre;
    overflow-x: auto;
}