You can use CSS in your emails, however many email clients do not implement the most current (or any) CSS standard.

Also note that most email clients automatically block external stylesheets. If you must use CSS, use only inline styles. Also, be sure to test your email thoroughly to make sure that your email is displayed properly in a few of the major email clients. This will greatly increase the chances that your contacts see the email you intended.

Things to keep in mind when using css in emails