 <p>See <a href="http://stackoverflow.com/questions/20495302/transparent-background-with-three-js">Transparent background with three.js</a> for transparent background rendering and then you can apply CSS gradients <a href="http://www.w3schools.com/Css/css3_gradients.asp" rel="nofollow">http://www.w3schools.com/Css/css3_gradients.asp</a> however it won't move with your scene unless you get really tricky.</p>