To uninstall fusiondirectory-theme-oxygen just follow these instructions.
sudo apt-get remove fusiondirectory-theme-oxygen
This will remove just the fusiondirectory-theme-oxygen package itself.
sudo apt-get remove --auto-remove fusiondirectory-theme-oxygen
This will remove the fusiondirectory-theme-oxygen package and any other dependant packages which are no longer needed.
If you also want to delete your local/config files for fusiondirectory-theme-oxygen then this will work.
sudo apt-get purge fusiondirectory-theme-oxygen
Or similarly, like this fusiondirectory-theme-oxygen
sudo apt-get purge --auto-remove fusiondirectory-theme-oxygen
|Maintainer||FusionDirectory packages maintainers group <firstname.lastname@example.org>|
|Description||Icon theme Oxygen for FusionDirectory|