Mobile devices feature overscroll and rubber banding, where you can scroll beyond the boundary of the contents and you will actually see the body. When we are in full view mode for a viewer like this, we expect to have a black background.

The top of the screen is also used to determine an implicit theme-color on certain devices (to style the chrome of the user-agent itself). As the top of our page changes in color depending on the image we load, we should set a specific theme-color ourselves.