How to set the white background on Portfolio Collection items to transparent (or black)? 🙏

Site URL: https://blueberry-maroon.squarespace.com/

In the Portfolio Collection overview every gallery grid item is showing a white background which I am not able to make it look transparent (or black). The hover color is the only thing that can be changed, but this is fine as is.

What is even stranger is that on the portfolio detail page the image is shown like it is meant to be, on a black background (transparent). 

What I tried so far is changing every possible color setting I could find and also applying all sorts of custom CSS, but with no luck so far. 🧐

The challenge is to change the white background color to transparent (or black) using custom CSS. I am a noob when it comes to such coding, so if somebody could help me to accomplish this using custom CSS, I would be forever grateful! 🙏

I am using SQSP 7.1 and the password is set to: monaco

UPDATE:
As it turns out, the image editor of SQSP is causing this issue. When cropping an image, the white background gets added. So for anyone encountering the same issue. Crop images BEFORE uploading into SQSP!

Hi,

Do you still need help on this?