style.css

html, body {
  margin: 0;
}

html {
  font-family: sans-serif;
  height: 100%;
}

body {
  overflow: hidden;
  height: inherit;
}

h1 {
  font-size: 1rem;
  letter-spacing: 1px;
  position: absolute;
  margin: 0;
  top: 10px;
  right: 10px;

  color: white;
  text-shadow: 0 0 4px white;
  z-index: 999;
}

control {
  position: absolute;
  margin: 0;
  top: 45px;
  right: 10px;
  z-index: 999;
}

control button,
setting div label button {
  background-color: white;
  box-shadow: 0 0 4px white;
  border: none;
  outline: none;
  border-radius: 15px;
  padding: 5px 15px;
  margin-left: 10px;
}

control button:active,
control button:focus {
  border: none;
  outline: none;
}

introduce,
setting {
  position: fixed;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  z-index: 999;
  background-color: rgba(0, 0, 0, .25);

  display: flex;
  justify-content: center;
  align-items: center;

  visibility: hidden;
}

introduce img,
setting div{
  position: absolute;
  top: 20%;
  width: 80%;
  max-width: 700px;
  background: white;
}

setting div label{
  display: block;
  height: 50px;
  line-height: 50px;
  margin: 20px;

  display: flex;
  justify-content: center;
  align-items: center;
}

setting div label button {
  background-color: rgba(0, 0, 0, .25);
  margin-right: 10px;
}