HTML Code

<!DOCTYPE html>

<html>

<head>

  <title>CSS :root Dark Mode Example</title>

  <style>

    :root {

      --bg: #ffffff;

      --text: #1e293b;

    }



    @media (prefers-color-scheme: dark) {

      :root {

        --bg: #0f172a;

        --text: #f1f5f9;

      }

    }



    body {

      font-family: system-ui, sans-serif;

      padding: 1.5rem;

      min-height: 100vh;

      margin: 0;

      background: var(--bg);

      color: var(--text);

      transition: background 0.2s, color 0.2s;

    }

  </style>

</head>

<body>

  <h1>Theme demo</h1>

  <p>Switch your OS to dark mode to see :root tokens update automatically.</p>

</body>

</html>

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CSS :root Selector - Dark Mode Tokens