Here's a bunch of css for you.... I started to play with it on your site, and ended up doing all the work for ya, LOL.

Replace stylesheet_header_menu.css with everything below.
Code:
body {
behavior: url(includes/csshover.htc);
}
/*this is the text color and background color when things are hovered over down the menu*/
div#dropMenu li a:hover, div#dropMenu li.submenu li.submenu:hover {color: #ffffff!important;background:#2b68ab;}
#dropMenuWrapper {
width:100%;
height:35px;
margin:0;
font-size:11px;
border-left:1px solid #999999;
}
div#dropMenu {
width:100%;
margin:0 auto;
text-align:center;
z-index:1000;
position:relative;
}
div#dropMenu ul {
margin: 0;
padding: 0;
}
div#dropMenu li {
position: relative;
list-style: none;
margin: 0;
float: left;
line-height: 1em;
width:12.5%;
}
div#dropMenu ul.level1 {
width:100%;
margin:0 auto;
text-align:center;
background:#ffffff; /*this is the background color of the main menu*/
height:35px;
z-index:1000;
}
div#dropMenu li:hover {}
div#dropMenu li.submenu {}
div#dropMenu li.submenu:hover {}
div#dropMenu li a {display: block; padding: 9px 0;text-decoration: none; text-transform:uppercase; color:#000000; text-align:center; border-right:1px solid #999999;height:17px;}
div#dropMenu>ul a {width: auto;}
div#dropMenu ul ul {position: absolute; width: 16em;display: none;}
div#dropMenu ul ul li {border-bottom: 1px solid #CCC; width:16em;}
div#dropMenu li.submenu li.submenu {}
div#dropMenu ul.level1 li.submenu:hover ul.level2,
div#dropMenu ul.level2 li.submenu:hover ul.level3,
div#dropMenu ul.level3 li.submenu:hover ul.level4,
div#dropMenu ul.level4 li.submenu:hover ul.level5 {display:block;z-index:1000;}
div#dropMenu ul.level2 {top: 35px; background:#999999;z-index:1000;}
div#dropMenu ul.level3, div#dropMenu ul.level4, div#dropMenu ul.level5 {top: 0; left: 16em; background:#F0555C}
div#dropMenu ul.level2 a {padding: 0.5em 0.25em;color: white; text-transform:none;} /* this is text color on drop-down submenu */
The above will make the menu fill your site (I changed the two instances of 750px to 100%), will make each item have a percentage width, gave each item a bigger height since you have a lot of drop downs and some of them would wrap onto two lines, and also adjusts color. You are actually using the categories version of the drop down menu which was developed by someone else, though I don't think he started a support thread, so its fine to ask questions here.
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