It is not best practice to include external stylesheets to render emails, in fact it won't even work for the vast majority of what you want to do as email clients ignore it or remove it.
To effectively render styled emails, you need to write your CSS inline, not even in the head of the email body. example:
<p style="color:#000;"></p>
Using inline styles will get you the best bang for your buck. Use the following resources at MailChimp as a guide: Mailchimp HTML Email Templates