Zen Theme gives you the advantage of customizing how your Showcase styling looks. Your available options are:
Showcase Background Gradient
Showcase Background Colour
Showcase Background Image & Repeat
This might be the more confusing for some but the theme loads a default green gradient backgroundimage that is 1 x 235 pixels and it repeats it from left to right along the top and will also have a background colour. This lets you set your own background image which can be a gradient or even a pattern. The background colour setting will also be important here because if you put an image or a slideshow into the Showcase area which has a height more than the default background image is, you need the bottom colour of the gradient to appear as though it's expanding downward. You can have your background repeat left to right like the default one or you can have it repeat side to side from top to bottom, regardless of height.