pre {
  border:1px solid #000;
  overflow-x:auto;
  background: #222;
  color: #fff;
  text-shadow: none;
}

.wrapper {
    max-width: 620px;
    padding: 0 20px 40px 20px;
    margin: auto;  
}

ul li img{
    margin: 0 auto;
}