style.css

body {
  margin: 0;
  font-family: monospace;
}

header {
  text-align: center;
  box-shadow: 0 0 8px 4px rgba(0,0,0,.1);
  padding: 32px;
}

header h1 {
  margin: 0;
}

section {
  max-width: 640px;
  margin: 0 auto;
}

nav {
  position: fixed;
  height: 100vh;
  top: 0;
  margin: auto 0;
  width: 64px;
  font-size: 32px;
  background-color: rgba(0,0,0,.7);
}

.icon-list {
  list-style: none;
  padding: 0px;
  position: absolute;
  top: 50%;
  transform: translateY(-50%);
}

.icon {
  padding: 8px;
  transition: all .2s;
}

.icon-norm {
  color: lightsteelblue;
}

.icon-current {
  color: darkseagreen;
}

.icon:hover {
  cursor: pointer;
  color: white;
  transform: scale(1.1);
  padding-left: 16px;
}

.example-code {
  margin: 0;
}

.indent {
  margin: 0 0 0 16px;
}

#previous-example {
  filter: grayscale(1) opacity(25%);
}

button {
  display: block;
  margin: 32px auto;
  color: white;
  width: 48px;
  height: 48px;
  border-radius: 50%;
  font-size: 32px;
  transition: transform .2s;
  outline: none;
}

button:hover {
  transform: scale(1.1);
  cursor: pointer;
}

button:disabled {
  filter: grayscale(1) opacity(5%);
}

#btn-back {
  background-color: steelblue;
}

#btn-next {
  background-color: mediumseagreen;
  line-height: 48px;
}

#question {
  text-align: center;
  margin: 64px 0 8px 0;
  color: steelblue;
}

#function-title {
  text-align: center;
  font-size: 32px;
  color: steelblue;
}

#example-box, #previous-example, .code {
  background-color: #EEE;
  padding: 16px;
  border-radius: 4px;
  border: 1px solid #DDD;
}


.example-title {
  margin: 0 0 8px 0;
  text-align: center;
  font-weight: bold;
  border-bottom: 1px solid #DDD;
  padding-bottom: 4px;
}

.comment {
  color: #BBB;
}