Theming HTML Mail sent through Mime Mail

The theme function attempts to suck in all of the theme's style-sheets. This "usually" looks OK, but often needs work.

The main issue is usually the fact that your theme's CSS files are designed to render the entire page. So your boiled-down HTML might look like:

However, theme_mimemail_message(), though it uses the same CSS, results in the following HTML:

Thus, your background color is black (per the body element) but your message content is never rendered as white, because the message layout doesn't include your #content id.

You have 3 options:

1. Add #mimemail-body to your CSS with appropriate defaults such as:
   
         #content, #mimemail-body {
        background-color: white;
      }
    
2. Create a mail.css file. Usually, you can copy your theme's style.css and begin making appropriate changes. In addition to changing the "offensive" declarations, you can also remove a lot of unused styles (such as sidebars) so that the overall message weight is much lower.
3. Override theme_mimemail_message() and do whatever you want. See http://drupal.org/node/55126 for information on how to do this.