Navigationsbutton gehen nicht

#nav_container{
position: absolute;
left: 50%;
top:140px;
margin-left: -490px;
width: 160px;
height:400px;
border:1px solid #FFFFFF;
overflow:auto;}
ul#nav{
width: 130px;
padding: 0px;
margin: 0px;}
li.nav_element{
list-style-type: none;
position: relative;
padding:5px;
width: 115px;}
li.nav_element a{
display: block;
width: 115px;
padding:4px;
font-weight: bold;
text-decoration: none;
text-align: left;
color:#F2F2F2;
background-color:#343434;
background-image:url(HIER BUTTON-URL_1);
border: 2px solid #000000;}
li.nav_element a:hover{
color: #000000;
background-color:#C9C9C9;
background-image:url(HIER BUTTON-URL_2);}