Hi Malane. Welcome to the forums.
The trick here is to wrap all of the content in a div, which has a set width and is centered. In that site, they've done it by putting all of the content in a div with an ID of "container":
... and then setting these styles:
all page content here
So the width is not set on the body element at all, which isn't a good idea anyway.
margin: 10px auto;
Have a look at that site in Chrome, right click anywhere on the page and select "Inspect Element". A window will open that shows you the HTML structure of the page and all the CSS that applies to each element. It's a great way to see how a page is constructed.