degov-8.x-2.0/modules/degov_html_mail/degov_html_mail.module

<?php

use Drupal\degov_common\Common;

/**
 * Implements hook_preprocess().
 */
function degov_html_mail_preprocess(&$variables, $hook, &$info) {
  // Add template suggestions and libraries implemented in this module.
  Common::addThemeSuggestions($variables, $hook, $info, [
    'module_name' => 'degov_html_mail',
    'entity_type' => 'swiftmailer',
    'entity_bundles' => [],
    'entity_view_modes' => [],
  ]);
  if ($hook == 'swiftmailer') {
    /** @var \Drupal\Core\Theme\ActiveTheme $theme */
    $theme = \Drupal::service('theme.manager')->getActiveTheme();
    // Check if there is mail.css exists in current active theme.
    $mailstyle = DRUPAL_ROOT . '/'. drupal_get_path('theme', $theme->getName()) . '/public/css/mail.css';
    if (is_file($mailstyle)) {
      // If yes include the content of the file into email message.
      $variables['css'] = @file_get_contents($mailstyle);
    }
    $variables['base_url'] = $GLOBALS['base_url'] . '/';
  }
}

/**
 * Changes the relative links in the email to absolute.
 *
 * Implements hook_mail_alter().
 */
function degov_html_mail_mail_alter(&$message) {
  // Attempt to convert relative URLs to absolute. Only apply this to parts
  // of the body which are markup objects.
  // Get Host of the site to have the links to start with it.
  $host = \Drupal::request()->getSchemeAndHttpHost();
  foreach ($message['body'] as &$body_part) {
    if ($body_part instanceof \Drupal\Component\Render\MarkupInterface) {
      $convertedString = \Drupal\Component\Utility\Html::transformRootRelativeUrlsToAbsolute((string) $body_part, $host);
      $body_part = \Drupal\Core\Render\Markup::create($convertedString);
    }
  }
}