styles.css

body {
  background: #303841;
  color: #eee;
  font-family: sans-serif;
  margin: 25px 0;
  text-align: center;
}

p, h1 {
  text-align: left;
}

h1 {
  font-size: 1.8em;
}

a {
  color: #00adb5;
  text-decoration: none;
}

.container {
  margin: 0 auto;
  max-width: 768px;
  position: relative;
  width: 100%;
}

.game {
  border-left: 1px solid #3a4750;
  border-top: 1px solid #3a4750;
  display: inline-block;
  margin: 25px 0;
  position: relative;
  width: 100%;
}

.game:after {
  background: rgba(0, 0, 0, 0.1);
  content: '';
  display: inline-block;
  height: 42.25%;
  left: 29.75%;
  position: absolute;
  top: 29.75%;
  width: 42.25%;
  z-index: 1;
}

.cell {
  border-bottom: 1px solid #3a4750;
  border-right: 1px solid #3a4750;
  float: left;
  padding-bottom: calc(2% - 1px);
  position: relative;
  width: calc(2% - 1px);
  z-index: 2;
}

.living {
  background: #00adb5;
}

.footer {
  color: #ccc;
  font-size: 0.85em;
  margin: 25px 0;
}

.hidden {
  display: none;
}