Cleanistic theme Help centralising the main menu.

Hello, I am trying to make the main menu align to the centre of the website and I was wondering if I could get some help on here, as I have had no luck so far. Below is the CSS relating to the menu:

/* TOP MENU
-------------------------------------------------------------- */
#top-navigation{
position:relative;
z-index:200;
padding:0px 0px 70px 0px;
font-family:"Century Gothic", Arial, Verdana;
height:0;
}

#topnav{
margin:0;
padding:0;
list-style-type:none;
overflow:visible;
  }
#topnav ul {
margin:0;
padding:0;
list-style-type:none;
  }

#topnav li {
margin-right:30px;
padding: 0 0px 0px 0px;
float:left;

width:auto;
text-transform:uppercase;
font-size:11px;
  }
#topnav a {
display: block;
padding:3px 0px 8px 0px;
text-decoration:none;
  }

#topnav li a:hover{
text-decoration: none;
display: block;
  }

#topnav li ul {
list-style: none;
position: absolute;
width: 130px;
top: 32px;
left:-999em;
padding:10px;
margin-left:-10px;
z-index:10;
filter: alpha(opacity=90); /* here you can set the opacity of box with text */
opacity: 0.9; /* here you can set the opacity of box with text */
/*background: url(images/bg-menu.png) repeat;*/
  }

#topnav li:hover ul, #topnav li.sfhover ul {  left: auto;  }

#topnav li ul li:hover ul {
left: 100%;
top: -11px;
margin-left:-10px;
}
#topnav li li {
  margin: 0;
  padding: 0;
  width: 130px;
  left: auto;
  height:auto !important;
  position:relative!important;
  }

#topnav li li a {
  width: 130px;
  line-height:18px;
  margin: 0;
  padding: 8px 0px 8px 0px!important;
  }

#topnav li li a:hover { padding: 8px 0px 8px 0px; }

#topnav li:hover, #topnav li.sfhover { /* prevents IE7 drop-down topnav bug (focus on a page element prevents nested topnavs from disappearing) */
  position: static;
  }

.lavaLamp {
  position: relative;
  height: 32px;
  padding: 0px 0px 0px 0px;
  overflow: hidden;
}

.lavaLamp li {
  list-style: none;
  float:left
}

#topnav li.back {z-index: 8;position: absolute;}
#topnav li.back:first-child{margin:0; left:0;}
.lavaLamp li a {  z-index: 10;}

And I think this is everything from the menu in the header.php file:

<div id="nav-menu">
<div id="myslidemenu" class="jqueryslidemenu">
<?php wp_nav_menu( array(
'menu_id'         => 'topnav',
'sort_column'    => 'menu_order',
)); ?>
</div></div><!-- end #top-navigation -->

Thanks.