What does your forum currently look like?
Seems to be in the center to me?
just wanted it to appear in the middle of the screen
To me it is in the middle, if you're talking about where all of the categories are.
Find In Global.css
#container
Add Width in it
width: 1000px;
Change 1000px; of your choice
Change in your global.css
#container {
color: #333;
text-align: left;
line-height: 1.4;
margin: 0;
min-width: 2000px;
to
#container {
color: #333;
text-align: left;
line-height: 1.4;
margin: 0;
min-width: 1000px;
and change wrapper style according to your need.