Making fast progress on my new Windows theme, having a lot of fun in fact!

But now I've reached a small roadblock. I've looked everywhere and still can't figure out how to change the white background color in some apps as shown in my screenshot. I want the bg color to match the dark grey of my window frame.

Is it possible for me to change that part of a window or is it defined by the programs themselves, making it impossible to reskin with WindowBlinds?

Arrow points to the section I'm trying to color: