layout css (header,nav 背景橫跨全螢幕)

#header_bg{
width: 100%;
height: 170px;
}
#header{
width: 990px;
height: 100%;
margin: 0px auto;
}
#nav_bg{
width: 100%;
height: 100%;
}
#nav{
width: 990px;
height: 30px;
margin: 0px auto;
}

#wrapper{
width: 950px;
padding: 20px 20px 0px 20px;
margin: 0px auto;
}
#content {
width: 100%;
min-height: 450px;
margin-bottom:10px;
}
#footer {
width: 100%;
float: left;
margin-top: 10px;
}