/usr/share/themes/NumixBlue/gtk-3.20/assets is in numix-blue-gtk-theme 0.54.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/themes/NumixBlue/gtk-3.20/../gtk-3.0/assets
/usr/share/themes/NumixBlue/gtk-3.20/../gtk-3.0/assets