The following allows you to apply a background to all Image Card blocks, including the stacked configuration. You'll have to look up the hex code for the color you want to use of course.
I then found that the text inside said cards was too close to the edges - something that isn't as noticeable when there is no background; it's especially apparent on mobile. I therefore added a bit of padding, which you can see in the code below (excuse my syntax).
