Simplenews css theming using mime mail
On admin/config/system/mimemail you can tell mimemail to try and use the site's stylesheet when sending an email. You'll have to make sure that your css selectors apply to the elements of your email message or adjust it appropriately.

Alternatively if you want more control over the styles, do not tell mimemail to use the site's stylesheet but instead create a file named mail.css in your theme. Mimemail will automatically use it when sending emails.

Also make sure your theme is set as "Theme to render the emails" under admin/config/system/mailsystem, otherwise you might get the wrong stylesheets when your emails get sent via cron, for example.