The Options → Background panel allows you to change the background for the game windows. You may define a solid color or a gradient of two colors, select a tiled background from built-in selections, or select a custom image of your own choosing from any image on your Mac.

The custom image may be tiled or stretched to fill the entire window per your preference.

Tip: When using the Select Color wells in the Solid Color(s) tab, click them to show the standard Apple color panel. Once the color panel is displayed, you click and then use the magnifying glass icon to select any color on your screen.