Can be done several different ways:

Look for this part of the css:

/* The main body of the entire forum. */
body
{
   background-color: #E5E5E8;
   margin: 0px;
   padding: 12px 30px 4px 30px;
}

Change the padding for the left and right side so they are the same number.
The two you want to change are the 2nd and 4th   (30px and 30px in the above example)

or

change margin: 0px;  to   margin: auto;
add:  width: 70%;    Change "70%" to the width you want it to be

or you can use pixels instead of percentage

change margin: 0px; to margin: auto;
add:   
width:  745px;   Just change 765px to the size you want it

30px is 30 pixels.   
It will put the left and right edges of the forum 30 pixels from the edge of the screen.
Thats the default - size you see here at support

If you want it wider change both of them (equally) to whatever amount you think you need it to be.
My avatar is 75 pixels wide if that helps ya