This is a modification of XFCE's Default-hdpi theme. I've made the outer border get the *background selected color* from current GTK theme only for active window. This way it makes current active window look like "selected", and it is especially useful with dark themes, because current (active) window really stands out in a very coherent way with the theme. For maximum visual coherence, you should also pick a theme whose CSD also include such border color for active windows (not many do; Oomox Numix-based themes do have this option). You can see a few examples of this theme's adaptability. It also respects shadows for window titles.
This decoration also mixes very well with Adwaita or derivatives, because of its button size.

Version 1.1 introduces bigger shadows, which is especially useful for dark themes, to help locate window borders better (although with the gtk-colored border it is already easier than in many themes)

Previews were made with:
- Adwaita-dark (this is the only one with version 1.1 (bigger shadows)
- Pho-Myrtus (check this great theme at https://www.pling.com/p/1240355)
- Pandora Glowing theme (a theme of my own, here: https://www.pling.com/p/1396322/)
Enjoy!