postoffice-1.0.x-dev/extensions/postoffice_compat_theme/tests/modules/hook_test/postoffice_compat_theme_hook_test.module

<?php

/**
 * @file
 * Primary module hooks for Postoffice Compat Fallback Test module.
 */

use Drupal\Component\Render\FormattableMarkup;

/**
 * Implements hook_mail().
 */
function postoffice_compat_theme_hook_test_mail($key, &$message, $params) {
  $element = [
    '#markup' => new FormattableMarkup('The currently active theme is: @theme', [
      '@theme' => \Drupal::theme()->getActiveTheme()->getName(),
    ]),
  ];

  /** @var \Drupal\Core\Render\RendererInterface $renderer */
  $renderer = \Drupal::service('renderer');
  $message['body'][] = $renderer->renderInIsolation($element);
  $message['subject'] = 'Current active theme';
}