plymouth-themes aren't visible in alternatives

[Jérôme]

I don't see the themes of the package plymouth-themes where running :
sudo update-alternatives --config default.plymouth

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: plymouth-themes 0.9.2-3ubuntu13.1
ProcVersionSignature: Ubuntu 4.4.0-62.83-generic 4.4.40
Uname: Linux 4.4.0-62-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.20.1-0ubuntu2.5
Architecture: amd64
BootLog: Error: [Errno 2] Aucun fichier ou dossier de ce type: '/var/log/boot.log'
CurrentDesktop: XFCE
Date: Tue Feb 7 23:49:33 2017
DefaultPlymouth: /usr/share/plymouth/themes/spinner/spinner.plymouth
InstallationDate: Installed on 2017-02-01 (6 days ago)
InstallationMedia: Xubuntu 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
MachineType: Dell Inc. Studio 1737
ProcCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.4.0-62-generic root=UUID=3c8cdb95-e7ae-4b71-a02d-5ad140a80282 ro quiet splash vt.handoff=7
ProcFB: 0 inteldrmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.4.0-62-generic root=UUID=3c8cdb95-e7ae-4b71-a02d-5ad140a80282 ro quiet splash vt.handoff=7
SourcePackage: plymouth
TextPlymouth: /usr/share/plymouth/themes/xubuntu-text/xubuntu-text.plymouth
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 02/11/2009
dmi.bios.vendor: Dell Inc.
dmi.bios.version: A05
dmi.board.name: 0P792H
dmi.board.vendor: Dell Inc.
dmi.board.version: A05
dmi.chassis.type: 8
dmi.chassis.vendor: Dell Inc.
dmi.chassis.version: A05
dmi.modalias: dmi:bvnDellInc.:bvrA05:bd02/11/2009:svnDellInc.:pnStudio1737:pvrA05:rvnDellInc.:rn0P792H:rvrA05:cvnDellInc.:ct8:cvrA05:
dmi.product.name: Studio 1737
dmi.product.version: A05
dmi.sys.vendor: Dell Inc.