You'll have to spend time learning how the stylesheets govern the displays. Follow Misty's advice and use Firefox with the developer toolkit, colorzilla and firebug addons.
Using these tools will allow you to find, edit and manipulate your stylesheet declarations OFFLINE, and to view the real-time effects changes will have on your displays.
This will save you hours of time and stop you from bashing your head against the wall.
For now:
Code:
.leftBoxContainer, .rightBoxContainer {
margin: 0em;
border: 1px solid #9a9a9a;
border-bottom: 5px solid #336633;
margin-top: 1.5em;
}
That green bar is border-bottom, and the declaration is saying:
"put in a border at the bottom, make it solid, 5 pixels deep and give it a puke green (#336633) colour."