Element Layers related to Background


I’m trying to figure out how to make the following elements invisible on the background gradient but visible on the black squares. I tried with z index but I cannot select the image z index but the whole section since the background is based on the whole section so when I change the z-index of the section it doesn’t affect the elements inside.

Any ideas on how to solve this and make the square, triangle and circle visible only in the black squares?

Many thanks in advance!

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