body {
  position: fixed;
  overflow: hidden;
  width: 100%;
  height: 100%;
  min-height: 100%;
  margin: 0;
  padding: 10px;
  line-height: 1.5em;
  background-color: #000;
  color: #fff;
  font-family: 'Avenir', Helvetica, Arial, sans-serif;
}

* {
  box-sizing: border-box;
  font-family: 'Avenir', Helvetica, Arial, sans-serif;
  -webkit-touch-callout: none;
  -webkit-user-select: none;
}

#buttons {
  position: absolute;
  top: 50%;
  left: 50%;
}

div.button {
  position: absolute;
  width: 100px;
  height: 100px;
  margin-top: -50px;
  margin-left: -50px;
  border-radius: 50%;
  opacity: 0.25;
}

div.button[data-index="0"] {
  top: -75px;
  left: -75px;
  background-color: green;
}

div.button[data-index="1"] {
  top: -75px;
  left: 75px;
  background-color: red;
}

div.button[data-index="2"] {
  top: 75px;
  left: -75px;
  background-color: yellow;
}

div.button[data-index="3"] {
  top: 75px;
  left: 75px;
  background-color: orange;
}

div.button.active {
  opacity: 1;
}