This is just a Xfce (Xfwm) window theme that represents a slight modification of the one included in the "DarK" GTK theme (https://www.xfce-look.org/p/1199936/, https://gitlab.com/sixsixfive/DarK/) and is meant as a variation to that. It replaces the original three colors of the window buttons with a single color for all three (#5F9EA0, Cadet Blue).