It's really easy so I thought I'd show you guys how to do it.
First of all, you need to wrap your comments in one big CSS class.
<table colspacing="0" colpadding="0" align="center" border="0" width="600">
<tr>
<td width="600" align="center" valign="top"">
<!--[COMMENTS=600]--> <!--[COMMENT_FORM=280,200]-->
</td>
</tr>
</table>
Find something along those lines in your template(going on default template, if you've modded the shit out your template, then just add the class to whichever table or div that goes around your comments), and give it a css class. I used the clas "comment" to add a class, in the table/td/div tag, add [code=]class="classname"[/code]. Good.
Now if you just do that, nothing'll happen unless the class you give it is already being use don your stylsheet, or if it's being used by DD's master style.
Now lets edit the CSC, most people have CSS on there template because it's defaulted onto DD.
<style type="text/css">
<!--

{CSS stuff goes here}

-->
</style>

Find this in the header section of your template, and if you don't have it, add it(get rid of the "{CSS stuff goes here}", though.
Now for the code itself;

.comment p{
background-color: #f8391d;
margin-bottom: 15px;
border: 2px solid #FFFFFF;
padding: 5px;
}

(change "comment" to whatever you named the class that you put around the comments)
This will create a red table with 15 px spacing between each table, and white borders.
Change the background color to whatever you want.
Changing the margin spacing will change how large the gaps between the comments are.
Change the border color, width, and style to whatever you want.
And padding would be the space between the comment and the border, 5 works for me.


Go ahead and screw around with it, it's easy to add a BG to the box.