I'm going to assume you want to add the gradient to one of the alts, since those are the classes that are all white on your site.
To do that, you need to add the gradient graphic using CSS in the style manager. This is done in the "First Alternating Color" and "Second Alternating" color sections in the style manager, via the "Background" field.
An example of what this might look like is this:
#xxxxxx url(images/gradients/xxxx.gif) repeat-x top
Of course, the "x"s would substitued with the proper information. The "#xxxxxx" would be the color, and the "xxxx.gif" would be the name of the gradient image. The "repeat-x" and "top" indicate the background image's positioning.
Hope this helps!