Remove black line from Masonry Gallery

I have a Masonry Gallery on the page noted in Private. (Halfway down the page, just above the red slanted content.) The icon images used are in grey. However, periodically a verticle black line will appear on some or all to the right of the icon images. I think this is due to there being a black background somewhere deep in the style behind the image. I’ve tried tracking it down so to change the color to match the button (so that the line no longer shows) but have failed to do so. Do you have a solution please?

Thanks!