This has the border scheme used in urukrama's Aeterna theme, with graphite colors and more traditional icons. The graphite color is picked to match the Rave-X Gtk theme:
I feel like this border width is ideal for visually distinguishing between windows. IMO, themes with no side borders or one-pixel borders lack usability--your eye can't easily pick out where one window ends and the next begins (unless you're using compositing with shadows).
Thanks for commenting and/or voting!