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.