This is the current code:
.menu-bottom-shadow {
background: url('images/sprite_h.png') repeat-x scroll left top #EEE;
}
Change it to this one (with your desired color):
.menu-bottom-shadow {
background: repeat-x scroll left top #000000 !important;
}
The line is an image images/sprite_h.png
so you either change the color in a graphic editing software such as Gimp to keep the gradient effect or you use the code above.
You can also check out the following link to get the gradient effect but I haven’t tested it.
http://www.w3schools.com/css/css3_gradients.asp