Most email readers do not render CSS properly if you use External or internal style sheets for you css. you need to use inline css instead. Also, when including a background image, you will need to specify the full URL for the image on your server, for example:
<div style="background-image:url(http://www.mydomain.com/images/image.jpg)">
In the Universal Email behavior, click the To button to enter the advanced To settings section, here you can set the Email Limits like Wait Time and Burst size.