Fervens Theme - increase width of content box

Hope someone can help :-)

I am using the fervens theme like this site (http://www.drupalfun.com)

I would like to change the layout so i can add a forum - as the white content box is too small - but only for the forum page.

I have tried to change the page.tpl.php page with no luck, I was only able to delete the two side bars to the right and that is it.

I have spent hours and hours trying to work out how to do it and read all the posts here. I downloaded Firebug and found that the size was determined by the following CSS but I cannot find it anywhere and I need it just for one page - the forum:

http://www.example.com/sites/default/files/css/d5937d9aa8a8ee495gbc81b19...

Here is the CSS in Firebug:

#primary li.active a:hover, #primary li.active-trail a:hover {
background:transparent url(/sites/all/themes/fervens/images/button-green-2.png) no-repeat scroll right center;
}
#main, body.sidebars #sidebar-left {
float:left;
}
body.sidebars #main.main-tl .main-tr .main-br .main-bl {
width:540px;
}
body.sidebar-left #main.main-tl .main-tr .main-br .main-bl {
width:600px;
}
body.sidebar-right #main.main-tl .main-tr .main-br .main-bl {
width:760px;

Thanks for your help in advance x