--- sites/all/modules/simplenews/simplenews.module	2009-06-11 10:51:18.000000000 +0200
+++ sites/all/modules/simplenews/simplenews_new.module	2009-06-11 10:51:04.000000000 +0200
@@ -2409,6 +2409,7 @@ function theme_simplenews_newsletter_sub
  * @see simplenews-newsletter-body.tpl.php
  */
 function template_preprocess_simplenews_newsletter_body(&$variables) {
+  if (file_exists(path_to_theme()."/simplenews.css")) drupal_add_css(path_to_theme()."/simplenews.css", 'theme');
 
   $variables['title'] = check_plain($variables['node']->title);
   $variables['body'] = $variables['node']->body;