With CSS, you can edit the opacity of a div, but everything within that div (such as a content) will also be set at the same opacity. What you could do it make a 50x50 square in photoshop, lower the opacity, make sure there is no background, and save that as a PNG file. Then, create your content div and use this:
#content {
width: [width];
height: [height];
background-image: url('[path to png]');
backrgound repeat: repeat;
margin: auto [to center the div];
margin-top: [how far div is from top of page];
border-radius: [# of pixels];
box-shadow: [add this in];
}
Please note that some of this is CSS3, so it will only show up in a compatible browser. Also, I added in the border-radius and box-shadow, looking at the link you gave. Remember, make the CSS3 stuff browser specific (e.g. -webkit-box-shadow)
Hope this helps!