First, thanks again for all your work on Retroarch. 
I’m trying to darken Material UI and make its scaling better for a desktop PC. I tweaked the values in menu/drivers/materialui.c to get something like this.
But I can’t find a way to make the background transparent (to see the running games between the menu, really nice to tweak shaders in real time).
Is there some values in the file to change to get it to work? Or does it imply additional coding?