been trying to figure this out...

This is going to be a little hard to explain. How do I make the forum fit the screen perfectly, like how xelation has (look in the attached image below).

[imported_plattopus]

In your CSS there should be something that looks like this:

body {
margin-top: 10;
margin-left: 10;
...
}

Remove the margins, and it should work.