a:hover {text-decoration: underline;}

Per the module instructions:

Messages are formatting using theme_mimemail_message($body), which includes all css files from the currently active theme and compresses the HTML version of the text.  Create a new theme function if your web stylesheets are too heavy for this.

There is no need to add functionality to the module... just add the theme_mimemail_messsage function into your template.php file. Use this function to point to any CSS file you like (I use something like mail.css), then create the CSS file with whatever styling you want.

Any chance of getting an example of how to add that PHP code? It would be great to have that in the readme file too for those who aren't fluent in PHP to set a custom css file

In case anyone stumbles on this, the mimemail checks for the mail.css in the mimemail module folder. If it doesn't find one, it will include all the default css files.

I made the mistake of adding the mail.css file in the theme folder, so I'm just documenting this.

tanjerine, you have to add the theme_mimemail_message function into your template.php file, then edit the function to point to a mail.css file located in your theme directory. You need to do the following:

1. Open the mimemail.module file
2. Copy and paste the theme_mimemail_message function into template.php
3. Change the "theme" part of the "theme_mimemail_message" name to be the name of your theme (for example, if your theme name is "abc", you would change the function name to "abc_mimemail_message")
4. Edit the function as you wish to point to any CSS file; I like to use a "mail.css" file located in my theme directory