Dynamic header with CSS only

HTML

<div id="container" style="height: 100%;">
    <div id="header" style="width:100% ">some stuff here...</div>
    <div id="content">ome other stuff here...</div>
</div>

CSS

body {font-family: sans-serif;}
img {background: #45719B;}
nav a {color: #45719B; font-weight: bold;}