When writing CSS most people use short hand because it saves space and thought.
For instance below says make a white backgorund with a horizontally repeating background image body_bgrd.png
background: #FFFFFF url(../images/body_bgrd.png) 0 0 repeat-x;
That works fine for your site's CSS but paste that into your HTML Mail template and your in for a surprise. :)
Instead you have to write the CSS out in long hand.
background-color:#FFFFFF;
background-image:url('http://www.triberising.com/files/images/body_bgrd.png');
background-position:0 0;
background-repeat:repeat-x;
Its a pain in know. But email clients and web browsers are different beasts. So to get complex CSS into your email templates write out each definition in the longer format.
If you don't know CSS long hand by heart w3schools.com is an invaluable resource.
For instance these background css properties can be learnt here:
http://www.w3schools.com/css/pr_background.asp