Actually, there are two background layers that are constantly visible. The top one is transparent and has light and dark images on it, and the bottom one is a g

Actually, there are two background layers that are constantly visible. The top one is transparent and has light and dark images on it, and the bottom one is a gradient with the same light and dark end colours. The top layer is fixed, the bottom layer absolutely positioned, so that the latter moves under the former as you scroll the page. Not exactly rocket science, and I guess we've never seen this before because it is way too distracting.