the problem is being caused in the general.css file. you have the following:
#outerWrapper #topNavigation ul li {
list-style: none;
display: inline;
padding: 2px 10px 2px 10px;
}
which is setting a 10px right padding and 10px left padding.
try changing:
padding: 2px 10px 2px 10px;
to:
padding: 2px 0 2px 0;
the short hand is top left bottom right, so:
padding: 2px 0 2px 0;
is the same as:
padding-top: 2px;
padding-left: 0;
padding-bottom: 2px;
padding-right: 0;