can't find file cas bg name

i wanted to change my cas bg for a more simple color, but the problem is: i just can't really find the file of the current bg i have

i have tried (almost) everything, searching for "cas", "bg", "back", "ground" "cas bg", in my files. tried to search for similar cas bg colors at chrome to maybe find the same file and discover the file name, but i could not

is there another way to remove the bg without searching for it in the files?