If you mean the lines above the menu/dropdowns, they’re actually created using an image file. You can see it in the stylesheet under “Main Navigation” in the “03. Headers” section, and the code looks like this…
nav#main-nav > ul:before, nav#main-nav > ul:after {
content: '';
width: 100%;
height: 6px;
display: block;
background: url("images/border.png") repeat-x top left;
You may notice that the image file (“images/border.png”) is called menu-border on your stylesheet – I replaced it with the double grey lines from the sidebar menu on mine. Unfortunately I’m not sure how to replace them with other pictures as I’ve actually got a thread about that very thing. If you delete that last line you can get rid of the grey lines, though. Hope that’s of some help.