"ProPhoto 4" > "Customize" > "Grids"
Certain grid styles allow you to customize the background of each individual grid item by setting a background image color and by uploading and customizing an optional background image.
Specifying a grid item background color or image usually requires also setting some grid item inner padding to make the grid items look right by adding some spacing between the edge of the grid items and inner content. See this tutorial for more.
Grid Item Background Color
By setting a grid item background color, you can make the grid item stand out from the grid context background, as pictured below:
Grid Item Background Image
More complex visual effects can be achieved by using a grid item background image either on its own, or in conjunction with a background color. In the below example, a grey-to-white gradient image has been uploaded and set to tile only horizontally across the bottom of the grid item, creating the effect that the grid item is fading into the background.