It actually has to do with another part of your code called – .full-width-content #content on line 975 in your css. It’s set to 960px, but if you add a border or border radius, you have to adjust your width of the content to make room for that border. Example, if your width is 960px, and you add 2px on the left and right (4px total), then you’ll need to decrease the width to 956px. Does that make sense?
Here is how I fixed it, I changed the 960px to 946px and the border shows up. You can adjust this more if you need to –
.full-width-content #content {
width: 946px;
}