How does the translucency work on this site?
http://grimmauld.legilimens.org/memorize.php

the background of a div is transparent automatically.
the image of the page is showing through the div that has no background color.
the creator even made sure of that by declaring transparent into it...

background-color: transparent;

Thank you.

Where did you find that information? I looked through the code when I viewed the source, but I didn't see it. Was it in an external stylesheet? If so, how can I look at that?

It's external

http://grimmauld.legilimens.org/sirius.css

view the source code of the page itself,
find the area you are looking at...

<div class="padfoot">
<p>Welcome to "Come Inside" the F

noticed that there was no css declared on the page itself for the class called "padfoot" but there is a link to an external page here...

<link rel="stylesheet" type="text/css" media="screen, projection" href="sirius.css" />

so in your browser, delete the last part of the URL address (the page name part) and type in the css page name. your browser should display the css page or open a text program to do so.