How to center <ul> <li> menù with CSS?

CSS
 
/* navigation */
#navigation {
	font: bold 11px Arial, Helvetica;
	text-transform: uppercase;
	margin: auto;
	padding-left: 0px;
	padding-top:0px;
	display: block;
	height: 56px;
	width: 860px;
	letter-spacing: 2px;
	background-image: url(images/navigation-bg.png);
	background-repeat: no-repeat;
	background-position: left top;
}
#navigation ul {
	float: left;
	line-height: 32px;
	list-style: none;
	margin: 0px;
	padding-top: 2px;
	padding-right: 15px;
	padding-bottom: 0;
	padding-left: 8px;
}
#navigation ul li {
	display: inline;
	padding: 0;
}
#navigation ul li img {
	padding-right: 5px;
	vertical-align: middle;
	border: none;
}
#navigation ul li a {
	color: #fff; /* need because it will use default link color */
	float: left;
	padding: 0 0px 0 0px; /* left 3 px because of left.png image */
	text-decoration: none;
	background: url(images/left.png) no-repeat left center;
	outline: none; /* Removed dotted lines around links */
}
#navigation ul li a span {
	display: block;
	float: none;
	padding: 0 10px 0 7px;
	background: url(images/right.png) no-repeat right center;
	outline: none; /* Removed dotted lines around links */
}
#navigation ul li a:hover {
	background: url(images/left-hover.png) no-repeat left center;
	outline: none; /* Removed dotted lines around links */
}
#navigation ul li a:hover span {
	background: url(images/right-hover.png) no-repeat right center;
	color: #fff;
	outline: none; /* Removed dotted lines around links */
}
#navigation ul li a:active {
	background: url(images/left-selected.png) no-repeat left center;
	outline: none; /* Removed dotted lines around links */
}
#navigation ul li a:active span {
	background: url(images/right-selected.png) no-repeat right center;
	color: #fff;
	outline: none; /* Removed dotted lines around links */
}
#navigation ul li a:focus {
	background: url(images/left-selected.png) no-repeat left center;
	outline: none; /* Removed dotted lines around links */
}
#navigation ul li a:focus span {
	background: url(images/right-selected.png) no-repeat right center;
	color: #fff;
	outline: none; /* Removed dotted lines around links */
}
#navigation .current_page_item a {
	background: url(images/left-selected.png) no-repeat left center;
	outline: none; /* Removed dotted lines around links */
}
#navigation .current_page_item a span {
	background: url(images/right-selected.png) no-repeat right center;
	color: #fff;
	outline: none; /* Removed dotted lines around links */
}
#navigation .current-cat a {
	background: url(images/left-selected.png) no-repeat left center;
	outline: none; /* Removed dotted lines around links */
}
#navigation .current-cat a span {
	background: url(images/right-selected.png) no-repeat right center;
	color: #fff;
	outline: none; /* Removed dotted lines around links */
}
 
HTML/PHP
 
<!-- NAVIGATION -->
<div id="navigation">
  <ul>
    <li><a href="<?php echo get_option('home'); ?>/" title="Home"><span>Home</span></a></li>
    <?php echo preg_replace('@\<li([^>]*)>\<a([^>]*)>(.*?)\<\/a>@i', '<li$1><a$2><span>$3</span></a>', wp_list_categories('echo=0&orderby=name&include=226&title_li=&depth=1')); ?>
    <?php $menu = array(
 
    'depth'       => 1,
 
    'show_date'   => '',
 
    'date_format' => get_option('date_format'),
 
    'child_of'    => 0,
 
    'exclude'     => '',
 
    'title_li'    => __(''),
 
    'echo'        => 1,
 
    'authors'     => '',
 
    'sort_column' => 'menu_order, post_title',
 
    'link_before' => '<span>',
 
    'link_after'  => '</span>',
 
    'exclude_tree'=> '' );
 
 
 
	wp_list_pages($menu);
 
	?>
  </ul>
  <div class="listcategories">
    <?php wp_dropdown_categories('show_option_none=Select category'); ?>
    <script type="text/javascript">
 
    var dropdown = document.getElementById("cat");
 
    function onCatChange() {
 
		if ( dropdown.options[dropdown.selectedIndex].value > 0 ) {
 
			location.href = "<?php echo get_option('home');
 
?>/?cat="+dropdown.options[dropdown.selectedIndex].value;
 
		}
 
    }
 
    dropdown.onchange = onCatChange;
 
</script>
  </div>
</div>
<!-- /NAVIGATION -->

                                              

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