Why does the eMail in my Rules Component use a theme that differs from page-theme?
You may want to check the result you get if you do NOT tell Mime Mail to use the site's stylesheet. And combine this with creating a file in your theme that is named mail.css. If you do that, then Mime Mail should automatically use that when sending emails. For more details on that, refer to its README.txt, i.e.:


  Email messages are formatted using the mimemail-message.tpl.php template.
  This includes a CSS style sheet and uses an HTML version of the text.
  The included CSS is either:
  
  
  the mail.css file found anywhere in your theme folder or
  the combined CSS style sheets of your theme if enabled.
  


Also make sure your theme is set as "Theme to render the emails" within admin/config/system/mailsystem. Without that, the stylesheets used by emails sent via cron (for example) may not look like you want them to look.