/usr/share/cairo-dock/themes/Default-Single/plug-ins/ is in cairo-dock-data 3.4.1-0ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/cairo-dock/themes/Default-Single/plug-ins/Animated-icons/
/usr/share/cairo-dock/themes/Default-Single/plug-ins/Animated-icons/Animated-icons.conf
/usr/share/cairo-dock/themes/Default-Single/plug-ins/Clipper/
/usr/share/cairo-dock/themes/Default-Single/plug-ins/Clipper/Clipper.conf
/usr/share/cairo-dock/themes/Default-Single/plug-ins/Dbus/
/usr/share/cairo-dock/themes/Default-Single/plug-ins/Dbus/Dbus.conf
/usr/share/cairo-dock/themes/Default-Single/plug-ins/GMenu/
/usr/share/cairo-dock/themes/Default-Single/plug-ins/GMenu/GMenu.conf
/usr/share/cairo-dock/themes/Default-Single/plug-ins/Recent-Events/
/usr/share/cairo-dock/themes/Default-Single/plug-ins/Recent-Events/Recent-Events.conf
/usr/share/cairo-dock/themes/Default-Single/plug-ins/dialog-rendering/
/usr/share/cairo-dock/themes/Default-Single/plug-ins/dialog-rendering/dialog-rendering.conf
/usr/share/cairo-dock/themes/Default-Single/plug-ins/dnd2share/
/usr/share/cairo-dock/themes/Default-Single/plug-ins/dnd2share/dnd2share.conf
/usr/share/cairo-dock/themes/Default-Single/plug-ins/icon-effect/
/usr/share/cairo-dock/themes/Default-Single/plug-ins/icon-effect/icon-effect.conf
/usr/share/cairo-dock/themes/Default-Single/plug-ins/illusion/
/usr/share/cairo-dock/themes/Default-Single/plug-ins/illusion/illusion.conf
/usr/share/cairo-dock/themes/Default-Single/plug-ins/logout/
/usr/share/cairo-dock/themes/Default-Single/plug-ins/logout/logout.conf
/usr/share/cairo-dock/themes/Default-Single/plug-ins/musicPlayer/
/usr/share/cairo-dock/themes/Default-Single/plug-ins/musicPlayer/musicPlayer.conf
/usr/share/cairo-dock/themes/Default-Single/plug-ins/quick-browser/
/usr/share/cairo-dock/themes/Default-Single/plug-ins/quick-browser/quick-browser.conf
/usr/share/cairo-dock/themes/Default-Single/plug-ins/rendering/
/usr/share/cairo-dock/themes/Default-Single/plug-ins/rendering/rendering.conf
/usr/share/cairo-dock/themes/Default-Single/plug-ins/shortcuts/
/usr/share/cairo-dock/themes/Default-Single/plug-ins/shortcuts/shortcuts.conf
/usr/share/cairo-dock/themes/Default-Single/plug-ins/showDesktop/
/usr/share/cairo-dock/themes/Default-Single/plug-ins/showDesktop/showDesktop.conf
/usr/share/cairo-dock/themes/Default-Single/plug-ins/stack/
/usr/share/cairo-dock/themes/Default-Single/plug-ins/stack/stack.conf
/usr/share/cairo-dock/themes/Default-Single/plug-ins/switcher/
/usr/share/cairo-dock/themes/Default-Single/plug-ins/switcher/switcher.conf