The main background is a PNG with the robots and zombies. It has black objects, gray objects and the rest is transparent. Beneath that PNG, there is a JPG gradi

The main background is a PNG with the robots and zombies. It has black objects, gray objects and the rest is transparent. Beneath that PNG, there is a JPG gradient that goes from gray to black. The PNG is fixed to the bottom, while the gradient isn't, so as you're scrolling, the gradient makes the background of the PNG appear different, revealing different objects.