Centering Navigation Bar in Purchased/Customized Theme- normal fixes don't work?

Hi All!

Sorry to join and immediately post a question– it’s been a few years since I’ve been up-to-date with my HTML and CSS, so now I’m having some trouble getting my navigation menu to align center. If I adjust the margins to auto, it centers the menu buttons, but bumps the menu background all out of whack.

#mainmenu-container {
		height:71px;
		width:1000px;
		padding:0 94px;
		margin:0 -79px;
		background:url(images/menu.png) left top no-repeat;
		position:absolute;
		top:0;
		z-index: 999;
	}
	#mainmenu {
		width:800;
		margin:0 auto;
		list-style:none;
	}
	#mainmenu li a {
		text-align:center;
		color:#fff;
		display:block;
		text-decoration:none;
		font-family:'Oswald';
		padding:18px 30px;
	}
	#mainmenu li {
		padding-left:7px;
		position:relative;
	}
	#mainmenu li a:hover{
		background:#91c000;
	}
	#header li ul {
		top:38px;
	}
	#header #mainmenu li ul {
		top:52px;
	}
	#header li ul .inner {
		background:#91c000;
		padding:0;
	}
	#header ul.children li, #header .sub-menu li {
		padding:0 !important;
		margin:0 !important;
		height:25px;
		line-height:25px;
	}
	#header ul.children li a, .sub-menu li a {
		color:#fff !important;
		padding:0 !important;
		background:none;
		height:25px;
		line-height:25px;
	}
	#header ul.children li a:hover, .sub-menu li a:hover {
		background:#a3cd23 !important;
		color:#fff !important;
	}
	#header .sub-menu .sub-menu {
		top:0 !important;
	}

My site’s live at mapkoza.com

I’m able to get the menu background image to move, or I can shove the margins in really tiny and force the menu on to two lines, but I can’t get the menu content centered on the menu background.

Thank you in advance for the help!