index.css

html,
body{
  height: 100vh;
  margin: 0;
  padding: 0;
  overflow: hidden;

  font-family: 'Roboto', 'Noto', sans-serif;
  font-size: 16px;
}

body {
  display: flex;
  flex-direction: column;
}

header {
  height: 56px;
  display: flex;
  flex-direction: row;
  align-items: center;
  padding: 0 20px;
  font-size: 20px;
  font-weight: 400;
  background-color: #000;
  color: #fff;
}

footer {
  display: flex;
  flex-direction: row;
  align-items: center;
  justify-content: center;
  padding: 4px 0;
}

.game-platform {
  flex: 1;
  position: relative;
  overflow: hidden;
}

.game-canvas {
  width: 100%;
  height: 100%;
  user-select: none;
}

.game-overlay {
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  flex-direction: column;
  opacity: 1;
  transition: opacity 0.25s ease;
}

.game-overlay.hidden {
  opacity: 0;
}

.closed {
  display: none;
}

.overlay {
  position: absolute;
  width: 100%;
  height: 100%;
  top: 0;
  left: 0;
  background-color: #ffffff;
  opacity: 0.60;
  z-index: 1;
}

.controls {
  padding: 40px;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  background-color: #ffffff;
  z-index: 2;
  position: relative;
}

.start,
.restart {
  cursor: pointer;
}

.demo {
  position: absolute;
  background: transparent;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
}