This is a mix of Plain (https://www.xfce-look.org/p/1016183/) and Metropolitan (https://www.pling.com/p/1015873) xfwm themes plus an addition from me for maximize and minimize buttons.
Both active and inactive window border colors are gotten from current GTK theme, so it'll blend very well with it. The previews are taken using Adwaita Dark Green and Nextwaita Dark SCALE GTK3 themes.
Also, window borders are thick enough so that resizing a window is not en exercise of marksmanship with the mouse.
Enjoy and thanks the original authors for these themes!