prevent-darktheme-flicker.html

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<script>
  !function () {
    try {
      var d = document.documentElement,
        n = 'data-theme',
        s = 'setAttribute';
      var e = localStorage.getItem('theme');
      if (e) {
        d[s](n, e || '');
      } else {
        d[s](n, 'dark');
      }

      if (e === 'light' || e === 'dark' || !e) d.style.colorScheme = e || 'dark';

    } catch (t) {
    }
  }();
</script>

<body>

</body>

</html>