Hi Currently breeze-gtk only installs the gtk theme associated with the gtk version found at build time. But gtk3 is smart enough to look for the right theme folder when they are all installed (gtk-3.0 would be used if nothing better, gtk-3.18 would be used if we are running gtk-3.18 or better, and so on). This would improve the upgrade path for our users, as it avoids having to rebuild breeze-gtk for the "right" gtk. Happy hacking,
Reported as bug 377568.