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Maybe this article help you http://www.outlook-apps.com/insert-html-to-outlook-emails/
You can use inline CSS within you HTML template and import as suggested
Inline CSS with less graphics is win win approach. I know writing inline CSS is a tedious and repetitive work but it works out ofr email. I mentioned less graphics because you've to consider visitors with mobile phone with limited bandwidth.
I won't lie but if I have been doing HTML Emails for1.7 years now and the simplest answer is you can only use inline css and also you need to use tables.