Full width “boxed”

Hi. Our theme is a “boxed” theme: the content area has a comfortable max-width, and if the window is wider, there is a background image on the body element that shows on the sides.

When I use a Parallax Scroll section, the scroll background image is stretched to the full width of the window, meaning that it is stretched unnecessarily on a very wide screen, and much of the far sides are cut off. It also means that I need to use a background image for the scrolling section that is as wide as the page window might be, necessitating a much larger file size, or else it looks pixelated on those very wide screens.

Is there anything that I can do in my CSS file to restrict the scrolling background images to the “boxed” width, rather than the full window width?