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When you place a mail.css file in your theme base directory, then Mimemail includes that instead of all the CSS files of your default theme. A handy feature to keep your emails clean and small.
When using the Zen theme however, things go wrong when you are used to place all your custom stuff in a Zen subtheme (like your should). Mimemail doens't look for a mail.css file in your Zen subtheme, it will only catch it if you place it in your base Zen theme directory.
I'm suggesting the following patch to have Mimemail's use of themes play nicer with the Zen theme. I know this is kind of a corner case and this patch isn't useful for a lot of people (not everyone uses the Zen theme). But it doesn't hurt for them and it will solve a lot of head aches for the unexperienced Zen/Mimemail/Drupal user.
When looking for a mail.css file, Mimemail now first looks in the Zen subtheme (if available), then in the normal (Zen base) theme. Nothing changes for non-Zen users, things get a lot better for those who do use Zen :)
So, include it if you like, otherwise: no hard feelings :)