Fix visibility of Windows

[Vittorio]

Hello there, I have an issue with these themes. As much as I really love the way they look, I really struggle when two or more windows overlap each other and because the theme is ever so flat in black it's really hard for me to distinguish between windows and clearly see their borders. I suggest implementing a very slight, yet efficient gradient in the windows so that people can clearly see where one window ends and the other is underneath.
Thank you