As this page says: http://css-tricks.com/using-css-in-html-emails-the-real-story/ you cannot link to an external style sheet, but you can add inline styles. so it looks like adding inline styles is a viable option.
Also, you cannot add styles to the head section, since some mail clients do not support it, again refer to http://css-tricks.com/using-css-in-html-emails-the-real-story/, so just concentrate on adding inline styles.