While editing my theme, I tried to use the parameter "bottom_window_darkness". It states values between 0 and 100 are allowed.
In the attached screenshot, the bottom_window_darkness setting was 10. The dark brown color in "Luftfahrt" is the chosen color for an active window. The green of "Straßenbahnen" is what happens to inactive windows of that color. But it does not work with player colors either, as "Finanzen" shows.
This is not limited to a setting of 10. No matter which number you pick, one out of three things will happen:
1) The inactive window does not change compared to the active window (eg. 0)
2) The inactive window becomes black (eg. 100)
3) The inactive window becomes that green if it's a custom (RGB) color. In this case, Player-Colors become black. (eg 10)
should be fixed with r8773