Untitled

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Document</title>

  <style>
    .darklight {
      background-color: #000;
      color: #fff;
    }

    .brightlight {
      background-color: #fff;
      color: #000;
    }
  </style>
</head>
<body>
  <h1 id="text"></h1>
  <script>
    window.addEventListener('devicelight', function(event) {
      var html = document.getElementsByTagName('html')[0];
      document.getElementById("text").innerHTML = event.value;
      if (event.value < 50) {
        html.classList.add('darklight');
        html.classList.remove('brightlight');
      } else {
        html.classList.add('brightlight');
        html.classList.remove('darklight');
      }
    });
  </script>
</body>
</html>