copypasta:


1. Make a small 2-px semi-transparent png image and use it as background:
.comment.odd, .comment.even{background:none;} #wrapper #container #center .right-corner .left-corner{background:url('http://img156.imageshack.us/img156/1014/creambg85.png')}
This will work 100% on every browser.

2.1 .comment.odd, .comment.even{background:none;} #wrapper #container #center .right-corner .left-corner{background:rgba(0, 0, 0, 0.8)}
This code might not work on older browsers.

RGBA: You'll have to use the RGB values of a colour for this. In this case
0 Red, 0 Green, 0 Blue, 0.8 Alpha = Black, 80% opacity
255 Red 255 Green 255 Blue, 0.2 Alpha = White, 20% opacity

((2.2 I'm not sure how to use this code with a background picture yet.))

3. .comment.odd, .comment.even{background:none;} #wrapper #container #center .right-corner .left-corner{opacity:0.8; filter:alpha(opacity=80)}
This code will also make all the content in your box semi-transparent.