index.css

@import url('https://fonts.googleapis.com/css?family=Fira+Sans');

:root, body {
  min-height: 100vh;
}

:root {
  font-family: 'Fira Sans', sans-serif;
  font-weight: normal;
  font-size: 112.5%;
  --color: #00a8d8;
}

body {
  max-width: 46rem;
  display: flex;
  flex-direction: column;
  justify-content: space-around;
  margin: 0 auto;
}

h1, h2, h3 {
  font-weight: normal;
  margin: .5rem 0;
}
h1 {
  font-size: 4rem;
}
h1 img {
  height: 1em;
}

p, blockquote {
  line-height: 1.7em;
}
blockquote {
  border-left: .2rem solid lightgray;
  margin-left: 0;
  padding: 1rem;
  color: darkgray;
}

a {
  color: var(--color);
}

code {
  background-color: cornsilk;
  padding: .3rem;
}

pre code {
  padding: 0;
}

header, main > section, footer {
  margin: 5rem 0;
}

header, footer {
  text-align: center;
}

.example {
  background-color: var(--color);
  padding: 1.5rem;
  color: white;
}

.example select, .example input, .example button {
  padding: .5rem;
  font-size: 1rem;
}

.example hr {
  border: 0;
  background-color: white;
  margin: 1.5rem -1.5rem;
  height: 2px;
}

.example ul {
  padding: 0;
  list-style: none;
}

.example li {
  padding: .5rem 0;
}

.example li.item {
  cursor: pointer;
  position: relative;
  padding-left: 2rem;
}

.example li.item:before {
  content: "";
  position: absolute;
  left: 0;
  height: 1rem;
  width: 1rem;
  border: 2px solid white;
  line-height: 1rem;
  text-indent: 2px;
}

.example li.done {
  text-decoration: line-through;
}

.example li.done:before {
  content: "✔"
}

a.button {
  border: 2px solid var(--color);
  display: inline-block;
  padding: .4rem 1rem;
  margin: .5rem;
  text-decoration: none;
}
a.button:first-child {
  margin-left: 0;
}
a.button img {
  height: 1rem;
  vertical-align: middle;
  margin-right: .5rem;
}
a.button:hover, a.button:focus, a.button:active {
  background-color: var(--color);
  color: white;
}
a.button span {
  text-decoration: underline;
}