.center {
  display: block;
  margin-left: auto;
  margin-right: auto;
}

html {
  filter: invert(100%);
  background-color: white; /* Important for transparent backgrounds */
}

body {
  max-width: 960pt;
  margin: auto;
  border: 1pt solid gray;
  text-align: center;
}

#s1, #s2, #s3, main {
    font-size: 100%;
    width: 232pt;
    height: 300pt;
    float: left;
    border: 2pt solid gray;
    margin: 2pt;
}

#s2 ul, #s3 ul {
    margin-left: 15pt;
    padding-left: 0;
}

header h1 {
  font-size:  300%;
  text-align: center;
}

header h2 {
  font-size:  100%;
  font-weight: bold;
  text-align: center;
}

main {
  margin-right: 0;
}

section, footer {
    font-size: 100%;
    clear: left;
}

#nav_menu ul {
    list-style-type: none;
    margin: 0;
    padding: 0;
    position: relative;
}

#nav_menu ul li {
    float: left;
}

#nav_menu ul li a {
    text-align: center;
    display: block;
    width: 240pt;
    padding: 8pt 0;
    text-decoration: none;
    background-color: blue;
    color: white;
    font-weight: bold;
    border-right: 0 solid white;
}

#nav_menu ul li a.lastitem {
    border-right: none;
}

/*   */