Hmm. Yeah, okay. A little beyond my pay grade, but I'll give it a shot.
Starting with the default theme, the first thing to do is to get the avatar back over to the left. Find the .commentlist li .avatar and change the float from right to left. While we're at it, lets adjust the border to give it that gray edge thing and curve the corners a bit (Firefox only, but what the heck):
.commentlist li .avatar {
float: left;
border: 3px solid #ccc;
padding: 2px;
background: #ccc;
-moz-border-radius:6px;
}
Next, you want to eliminate the special background colors on the .commentlist bits, along with the lines between them. There's some business with the .alt that you will want to check for too. Just find and eliminate the css that's giving it those background colors.
We want to jazz up the comment text a bit, so let's pad it around a bit and give it a new background color, as well as some slightly less curvy corners:
.commentlist p {
margin-top:30px;
background: #ccc;
padding:10px;
-moz-border-radius:4px;
}
If you want a gradient like in your example, you'll probably need to use an image for the background instead. Same goes for the up arrow thingy.
That text seems awfully close to the avatar though. Let's push it right 10 pixels or so:
.commentlist cite, .commentmetadata a {
margin-left:10px;
}
I think you get the idea at this point. Just move things around until you get the look you want.
Note that doing this is all a lot simpler with a good tool. I like to use the Web Developer plugin for Firefox. It lets me edit CSS on a live webpage and see the changes as I make them. Then I can copy/paste my changes into the CSS file to make them permanent.