Twenty Eleven Content Border

Where in the CSS or otherwise would I be able to add a border / box around the whole content section with a drop shadow? Ideally I’d like to use an image / sliced up image for the border with drop shadow.

i’d like to do something similar to this. With the content surrounded by a border with a drop shadow.

I’m competent in actually implementing, but looking for exactly where to edit in the Twenty Eleven theme.

Thanks!