style.css

td.navigation_bar {
  border-right-style: dashed;
  border-right-color: brown;
  border-right-width: 2px;
  vertical-align: top;
}

td.navigation_bar ul {
  list-style-type: none;
  margin: 0;
  padding: 0;
  width: 150px;
  background-color: #f1f1f1;
}
td.navigation_bar ul li a {
  display: block;
  color: #000;
  padding: 8px 16px;
  text-decoration: none;
}

td.navigation_bar ul li a:hover {
  background-color: #555;
  color: white;
}

div {
  margin: 0 auto;
  font-family: Roboto;
  position: relative;
}

div.title {
  width: 35%;
  text-align: center;
  border-bottom: 1px solid green;
}

div.intro {
  width: 40%;
  padding-left: 2vw;
  border-left: green groove;
  background-color: #d3ffed;
  font-style: italic;
  font-size: 20px;
}

div.main {
  width: 60%;
  font-size: 20px;
}

div.definition {
  width: 40%;
  padding-left: 2vw;
  border-left: brown groove;
  background-color: blanchedalmond;
  font-style: italic;
  font-size: 20px;
}

img.equation {
  vertical-align: middle;
}

img.single_image {
  display: block;
  margin-left: auto;
  margin-right: auto;
  width: 70%;
}

div.hub {
  padding: 70px 0;
  border: 3px solid green;
  text-align: center;
  position: absolute;
  top: 40%;
  left: 10%;
  width: 80%;
}

div.hub ul {
  list-style-type: none;
  margin: 0;
  padding: 0;
  width: 80%;
  background-color: #f1f1f1;
  margin: 0 auto;
  font-size: xx-large;
}
div.hub ul li a {
  display: block;
  color: #000;
  padding: 8px 16px;
  text-decoration: none;
}

div.hub ul li a:hover {
  background-color: #555;
  color: white;
}