html, body {
  margin: 0;
  padding: 0;
  overflow: hidden;
  font-family: Arial, sans-serif;
  background: #000;
  height: 100%;
}

#menu, #briefing {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  background: rgba(30,30,30,0.9);
  color: #fff;
  padding: 20px;
  border-radius: 10px;
  text-align: center;
  border: 2px solid rgba(255,255,255,0.2);
  z-index: 10;
}

#menu h2, #briefing h2 {
  margin-bottom: 15px;
}

#menu button, #briefing button {
  display: block;
  width: 200px;
  margin: 10px auto;
  padding: 10px;
  font-size: 16px;
  background: rgba(255,165,0,0.3);
  border: 1px solid orange;
  color: #fff;
  border-radius: 5px;
  cursor: pointer;
}

#menu button:hover, #briefing button:hover {
  background: rgba(255,165,0,0.6);
}

#hud {
  position: fixed;
  top: 10px;
  left: 10px;
  color: #fff;
  font-size: 18px;
  z-index: 5;
}

#gameCanvas {
  position: absolute;
  top: 0;
  left: 0;
  z-index: 1;
  width: 100%;
  height: 100%;
}