.center {
    margin: auto;
    width: 50%;
}
@media only screen and (max-width: 720px) {
    /* For mobile phones: */
    .center {
    width: 100%;
}
li.nav {
    display: inline;
    margin-right: 10px;
}
li.nav a {
  display: block;
  padding: 8px;
  background-color: #dddddd;
}
.align-left {
    float: left;
    margin-right: 10px;
}
.medium {
    width: 250px;
    height: 250px;
}
}