Gtk themes like Adwaita use the :backdrop css selector to appropriately dim their highlight and selection colors when an application window loses focus. This visual cue keeps the user from mistakenly typing (or copying, pasting, etc.) in the wrong window. Unfortunately, it doesn't work on Xfce, because xfwm doesn't support the _NET_WM_STATE_FOCUSED window manager hint.
This has been biting me more and more as the applications I use migrate from gtk2 to gtk3. It would be quite helpful if xfwm added support for the hint.
Marking as duplicate.
*** This bug has been marked as a duplicate of bug 11067 ***