As far as I understand, the settings made in Tools -> Customized are stored in file registrymodifications.xcu in your user profile and hence the only way to get back the defaults is to force LibreOffice to recreate the file (which gets done, if you delete the file).But this will also reset almost all other settings, you made through Tools -> Options. It is also possible to edit the file, but I do not endorse to do so. Take the Save-your-configuration-and-restore-approach.