How do I give pages a different background in Holder without hiding my background image?

Hi. I want to have the background image show on the left and right of the wrapper area, but beneath my content I want another color or image. I tried all the Holder options and the only one that applied was the middle strip color, but that hid my background on the left and right, except if you scroll down far enough, then it shows my background image.

How do I make it so that there's a layer of something underneath the middle vertical strip of screen so that my text is visible? Ideally I'd like a partially transparent solid color so that the background image is somewhat visible through it, yet the content fully readable.

Thanks.