Hacking a theme’s css

With the help of someone far more skilled at css than I, we’ve hacked my new theme to widen the post display area. But if you look on my main pg. here you’ll see a thin grey line currently running down the (slightly off-) center of the pg., which marks where the old right-side border used to be. That line needs to be moved farther right to the new border so it separates the right margin of the post display area fr. the right sidebar.

How would I do this?