spec-main/fixtures/pages/css-transparent.html

<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
    <title>test-color-window</title>
    <style>
      body {
        background: red;
      }
    </style>
  </head>
  <body id="body">
    <script>
      const { ipcRenderer } = require('electron');

      const observer = new MutationObserver((mutationList, observer) => {
        mutationList.forEach(({ type, attributeName }) => {
          if (type === 'attributes' && attributeName === 'style') {
            ipcRenderer.send('set-transparent');
          }
        });
      });

      observer.observe(document.body, { attributes: true });

      document.addEventListener('DOMContentLoaded', event => {
        document.body.style.background = 'transparent';
      });
    </script>
  </body>
</html>