How do you widen a forum when a sidebar is used

I’ve been searching for a very long time and all I can find is how to hide a sidebar, or make a full width page. I need to keep my sidebars in my forums, but they are causing the forums to be rather small to the point where it’s almost hard to read. I’ve tried this:

#sidebar-left .widget, #sidebar-right .widget {
margin: 0 0 1em;
}

But I think I might need something else to force the forum wider because the code above did nothing for me.