How to put Gradient Color, instead of solid color, in horizontal Page/Catgry Menu Bar

Is it possible to assign a gradient color bar image to the Background behind the page/category menu items, in the horizontal menu bar?

I already have some good gradiated color images the same height as the current menu bar, which I liked to put there instead of the solid color.

The Page or Cat options has a space for you to change the html color # for the background, but you can't insert anything in there like an image.jpg, so where would you go (in the CSS?) to change this to an image you select, while still having the pages/categories work as links.

Alternately, is it possible to overlay the menu choices over the header image, while still having the pages/categories work as links?

Thanks
Rhythm