To add borders to elements using 960 Grid, create divs under the elements 960Gid and apply borders to them.
960Grid applies margins, borders, padding to all the elements it creates. So intentions of overriding these may not work out.
The comment here explains it in other words: Also for anyone stumbling here, be careful about something like “border:1px solid;” on the divs, they will increase the width of the div by +2 px and it will cause the same symptom described. You run into the same problem with padding. The solution for that is to add divs inside your grid_x divs, and apply the border or padding to those inner divs. – Purrell Jan 24 ’11 at 2:55