Use different color for preview frame and window list background

Both the window list background and the frame around previews use the text background color (white for me). In most cases, the window they are displayed on top of has the same background color (terminal window, editor, pdf reader, thunar file list) (also white).

This makes the window list and the preview hard to see and hard to distinguish from the background, because they don't have any visible border at all: The window list background is white, most of the window preview itself is white, the border around the window list and the preview is white, and the window behind the window list and preview is also white (I have no shadows and all X compositor effects turned off).

Ideally, the background color used for the window list and the border around the previews should be configurable. At least, the window list and preview frame should use the dialog background color (grey), not the text background color.