The menu items can have a space (padding) between their borders and their group's borders (edges). This space is filled by their group's background, which can be a color or a background image (texture, etc.).
In this example, there is a padding of 10 pixels which is filled by a background image ("OLDWOOD.jpg").
You can adjust the padding of each side all together or even separately!
(see image below for different padding per side)
Download this AllWebMenus project file.