In stylesheet_header_menu.css, find this
Code:
div#dropMenu {
	position:relative;
	width: 100%;
	height: 25px;
	margin-top: -4.5px;
	/*margin-top: -1px;  */
    text-align:left;
	z-index: 1000;
	font-family: Lucida Sans;
	}
and replace the width with padding-left:
Code:
div#dropMenu {
	position:relative;
	padding-left: 2.5em;
	height: 25px;
	margin-top: -4.5px;
	/*margin-top: -1px;  */
    text-align:left;
	z-index: 1000;
	font-family: Lucida Sans;
	}
The width needs to go because explicitly defining width forces margins or padding to be added to that width (i.e. 100% + 2.5em, or wider than the full width). Divs will be 100% width by default, and letting them be natural lets the margin/padding fit inside the container.