CSS-Problem beim Zentrieren

#extraDiv1 {display: none;}
#extraDiv2 {display: none;}
#extraDiv3 {display: none;}
#extraDiv4 {display: none;}
#extraDiv5 {display: none;}
#extraDiv6 {display: none;}
#pre_header {display: none;}
#post_header {display: none;}

#nav_heading{display: none;}
div.header{display: none;}
h1#title{display: none;}
h2#title span {display: none;}

a{
color:#000000;
text-decoration: none;
font-size:13px;}

a:hover {
color: #4A9AF6;
font-size: 13px;
text-decoration: none;}

a:link {
color: #4A9AF6;
font-size: 13px;
text-decoration: none;}

a:active {
color: #4A9AF6;
font-size: 13px;
text-decoration: none;}

a:visited {
color: #4A9AF6;
font-size: 13px;
text-decoration: none;}


body {
background-color:#000000;
left: 50%;
background-image:url();}


#header_container {
left: 50%;
margin-left:200px;
padding: 5px;
width: 1000px;
height:200px;
background-color:#343434;
background-image:url();
color:#FFFFFF;
font-size:13px;
border: 1px solid #FFFFFF;
-moz-border-radius: 10px;}


#content {
left: 50%;
margin-left:380px;
margin-top:-315px;
width: 820px;
color: #000000;
padding: 5px;
padding-bottom: 20px;
font-size: 14px;
font-family: Arial;
background-color:#045687;
background-image:url();
border: 1px solid #FFFFFF;
-moz-border-radius-topright: 10px;
-moz-border-radius-topleft: 10px;}


#nav_container {
left: 50%;
margin-left: 200px;
margin-top:20px;
width: 160px;
height: auto;
background-color:#C1490E;
background-image:url();
border:1px solid #FFFFFF;
color:#XXXXXX;
-moz-border-radius: 10px;}

ul#nav{
width: 130px;
padding: 0px;
margin: 0px;}

li.nav_element{
list-style-type: none;
position: relative;
padding-bottom:15px;
padding-left:2px;
border: 1px;
width: 110px;
height: 20px;
text-align: left;}

li.nav_element a{
display: block;
padding:3px;
width: 115px;
height: 20px;
margin: 10px;
color: #F2F2F2;
font-weight: bold;
text-decoration: none;
background-color: #xxxxxx;
border: 0px solid #ffffff;
text-align: left;}

li.nav_element a:hover{
display: block;
color: transparent;
font-weight: bold;
text-decoration: none;
background-color: #XXXXXX;
color:#000000;
border: 0px solid #ffffff;
text-align: left;}

#content_footer{
left: 50%;
margin-left: 380px;
margin-bottom:0px;
margin-top: -21px;
height: 20px;
padding:5px;
width: 820px;
font-family:Arial;
font-size:12px;
color:#FFFFFF;
background-color:#656565;
-moz-border-radius-bottomright: 10px;
-moz-border-radius-bottomleft: 10px;
border: 1px solid #ffffff;}