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.
Previews were made with:
- 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/)