I'm trying to solve this in the following jsfiddle: https://jsfiddle.net/0u9tqfr2/1/
body { background-image: url(...); }
#header { background-color: aqua; width: 800px; }
#sidebar { background-color: yellow; width: 200px; float: left }
#content { background-color: lightgray; width: 500px; }
