Adding one border to two content areas

I need some help with my blog at https://www.san-francisco-personal-injury-attorney.org/. I am modifying the file(s) directly using FTP.

I am trying to create a 7 px border that encompasses all of the white content area down to the footer. I’m trying to fix this via modifying the file(s) directly, but I cannot figure out how.

It looks as though the footer and the content area don’t match up, as far as width is concerned, and I modified the header and the footer through the CSS file to be 940px, but I’m not sure where to find the width of the content.

I’m guessing that once the border is added, all of the elements aren’t going to be flush against it due to them being slightly different widths.

Thank you for your help!