Theme Edition - Publication Cover

Hi There

I’m using the EDITION theme for my blog. I need to remove the grey overlay that dulls/greys the Publication cover image. I found a Code Injection suggestion on the forum and tried to use it to fix my problem. But it did not help. I used the below code:

.is-cover::before { background-color: transparent }

Please refer to the attached picture, the preview on the right is grey, I uploaded a white cover, on the left.