:Instructions
        <p>When formatting an email message with a given <code>$module</code> and <code>$key</code>, <a href="https://www.drupal.org/project/htmlmail">HTML Mail</a> will use the first template file it finds from the following list:</p>
        <ol style="list-style-type: decimal;">
          <li><code>htmlmail--$module--$key.html.twig</code></li>
          <li><code>htmlmail--$module.html.twig</code></li>
          <li><code>htmlmail.html.twig</code></li>
        </ol>
        <p>For each filename, <a href="https://www.drupal.org/project/htmlmail">HTML Mail</a> looks first in the chosen <em>Email theme</em> directory, then in its own module directory, before proceeding to the next filename.</p>
        <p>For example, if <code>example_module</code> sends mail with:</p>
        <pre>
          <code>\Drupal::service('plugin.manager.mail')->mail("example_module", "outgoing_message" ...)</code>
        </pre>
        <p>the possible template file names would be:</p>
        <ol style="list-style-type: decimal;">
          <li><code>htmlmail--example_module--outgoing_message.html.twig</code></li>
          <li><code>htmlmail--example_module.html.twig</code></li>
          <li><code>htmlmail.html.twig</code></li>
        </ol>
        <p>Template files are cached, so remember to clear the cache by visiting <u>admin/config/development/performance</u> after changing any <code>.html.twig</code> files.</p>
        <p>The following variables available in this template:</p>
        <dl>
        <dt><strong><code>message.body</code></strong></dt>
        <dd>
          <p>The message body text.</p>
        </dd>
        <dt><strong><code>message.module</code></strong></dt>
        <dd>
          <p>The first argument to <a href="https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Mail!MailManager.php/function/MailManager%3A%3Amail"><code>MailManager::mail</code></a>, which is, by convention, the machine-readable name of the sending module.</p>
        </dd>
        <dt><strong><code>message.key</code></strong></dt>
        <dd>
          <p>The second argument to <a href="https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Mail!MailManager.php/function/MailManager%3A%3Amail"><code>MailManager::mail</code></a>, which should give some indication of why this email is being sent.</p>
        </dd>
        <dt><strong><code>message.message_id</code></strong></dt>
        <dd>
          <p>The email message id, which should be equal to <code>"{$module}_{$key}"</code>.</p>
        </dd>
        <dt><strong><code>message.headers</code></strong></dt>
        <dd>
          <p>An array of email <code>(name =&gt; value)</code> pairs.</p>
        </dd>
        <dt><strong><code>message.from</code></strong></dt>
        <dd>
          <p>The configured sender address.</p>
        </dd>
        <dt><strong><code>message.to</code></strong></dt>
        <dd>
          <p>The recipient email address.</p>
        </dd>
        <dt><strong><code>message.subject</code></strong></dt>
        <dd>
          <p>The message subject line.</p>
        </dd>
        <dt><strong><code>message.body</code></strong></dt>
        <dd>
          <p>The formatted message body.</p>
        </dd>
        <dt><strong><code>message.language</code></strong></dt>
        <dd>
          <p>The language code for this message.</p>
        </dd>
        <dt><strong><code>message.params</code></strong></dt>
        <dd>
          <p>Any module-specific parameters.</p>
        </dd>
        <dt><strong><code>template_name</code></strong></dt>
        <dd>
          <p>The basename of the active template.</p>
        </dd>
        <dt><strong><code>template_path</code></strong></dt>
        <dd>
          <p>The relative path to the template directory.</p>
        </dd>
        <dt><strong><code>template_url</code></strong></dt>
        <dd>
          <p>The absolute URL to the template directory.</p>
        </dd>
        <dt><strong><code>theme</code></strong></dt>
        <dd>
          <p>The name of the <em>Email theme</em> used to hold template files. If the <a href="https://www.drupal.org/project/echo">Echo</a> module is enabled this theme will also be used to transform the message body into a fully-themed webpage.</p>
        </dd>
        <dt><strong><code>theme_path</code></strong></dt>
        <dd>
          <p>The relative path to the selected <em>Email theme</em> directory.</p>
        </dd>
        <dt><strong><code>theme_url</code></strong></dt>
        <dd>
          <p>The absolute URL to the selected <em>Email theme</em> directory.</p>
        </dd>
        <dt><strong><code>debug</code></strong></dt>
        <dd>
          <p><code>TRUE</code> to add some useful debugging info to the bottom of the message.</p>
        </dd>
        </dl>
        <p>Other modules may also add or modify theme variables by implementing a <code>MODULENAME_preprocess_htmlmail(&amp;$variables)</code> <a href="https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Render%21theme.api.php/function/hook_preprocess_HOOK">hook function</a>.</p>