I would like SimpleNews use another theme (Bartik for example) than the admin one (Seven). I configured MailSystem to use "Bartik" as theme to render emails, as written in SimpleNews README.txt:
The template files are self documented listing all available variables. Depending on how the mails are sent (e.g. how cron is triggered), either the default or the admin theme might be used, if one has been configured. To prevent this, Simplenews supports the mail theme setting from the mailsystem module (http://drupal.org/project/mailsystem). Install it, choose the mail theme and the newsletter templates from that theme will be used no matter which other themes are enabled.
In the simplenews.module file, I got the line:
2674. $theme = function_exists('mailsystem_get_mail_theme') ? mailsystem_get_mail_theme() : path_to_theme();
$theme is evaluated as "theme/bartik". Great, but it keeps use the "simplenews-newsletter-body.tpl.php" file in "theme/seven" directory.
I struggle with that for weeks now, any help would be welcome!