Twenty Seventeen theme – how to stop main page parallax fades

How do I stop the main page / parallax images from fading (darkening)?

Using the twenty-seventen theme, and using the full screen images on the main page, as you pull the browser slider down to slide from image to image (section to section), the images fade (darken) a little. How do I stop this darkening.

I want everything else to operate as is, just no fading / darkening of the images as the scroll bar is pulled down.

Thanks.
– Dan