+1 On a dual-monitor setup I'm also getting this effect on the window without focus. The following screengrab is not how it looks to the naked eye: When I opened the screengrab in an image editor the icon background is transparent with white icon.. actual appearance to the eye is black background white icon like this:

Thanks for the feature request @Tomáš-Rus - we haven't done this yet (would need to implement the pref pane as you say.