Add support for mail.css

@liquidcms it's not working for me, I get a WSOD when I try to send an example mimemail and this in the log:

Error: Class "LanguageInterface" not found in mytheme_preprocess_mimemail_message() (line 41 of /var/www/html/name/web/themes/custom/mytheme/mytheme.theme)
…

@portulaca It sounds like you need a 'use' statement for LanguageInterface in the file that you have added the code from #10 to. I can't recall the exact place but something of the form:

use \Drupal\Core\...\...\LanguageInterface;

Search existing code for examples.

@rivimey thank you!

It's

use Drupal\Core\Language\LanguageInterface;
use Drupal\Core\Url;

It's working now, although I'm getting a double slash in the CSS link path https://example.com//themes/custom/…

Should the variable be without it?

$variables['mail_css'] = $base_path . 'themes/custom/my_theme/css/mail.css';

@malcomio: Are you planning on working on this issue?

In the project where we're using mimemail, we ended up adding a preprocess in our theme, as suggested in #10, and I haven't had a chance to make progress on a contributed version.

We used the preprocess theme suggestion as in #10. This successfully includes the mail.css file.

Hi. The method from #10 is adding a link to the mail.css in the mimemail-message template. From what I saw it did not work in all cases. To actually add the CSS into the template, I have used the below code:

/** @var \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler */
  $theme_handler = \Drupal::service('theme_handler');
  $theme_path = $theme_handler->getTheme('YOUR_THEME')->getPath();

  $css = FALSE;
  // Assuming mail.css is in a /css subdirectory where all other css is.
  $css_path = DRUPAL_ROOT . '/' . $theme_path . '/css/mail.css';

  try {
    $css = file_get_contents($css_path);
  }
  catch (Exception $exception) {
    Drupal::logger('YOUR_THEME')->error(t('Failed to get contentns of mail.css file while using mimemail-message template. Exception message: @exception', [
      '@exception' => $exception->getMessage(),
    ]));
  }

  if ($css) {
    $variables['css'] = $css;
  }

With this code there is not need to modify the mimemail-message twig template unless you want to add some additional markup in it.

I hope this helps someone.