Crave: How to make zoomed product image modal transparent on the sides

[asia-acorn]

I have made my product modal smaller, so when people click on a product image from the product page, the images are not so big.

Now I would like to make the white areas that are on either side of the grey area semi-transparent, so that users know they are still on the product's page and don't have to click the 'back' button to go back to the product. I cannot find anywhere in code to change this, and I need help please.

I am using the Crave theme