/usr/share/themes/Greybird/xfwm4/bottom-inactive.xpm is in greybird-gtk-theme 3.22.8-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/themes/Greybird/xfwm4/bottom-active.xpm
/usr/share/themes/Greybird/xfwm4/bottom-active.xpm