How to install custom plymouth in cubic?

Question

I am trying to install my custom plymouth theme doing this:

 sudo update-alternatives --install /usr/share/plymouth/themes/default.plymouth default.plymouth /usr/share/plymouth/themes/mytheme/mytheme.plymouth 200
 sudo update-alternatives --set default.plymouth /usr/share/plymouth/themes/mytheme/mytheme.plymouth
 update-initramfs -u
 update-grub

My custom theme shows up when i test it with:

 sudo plymouthd && sudo plymouth --show-splash && sleep 20 && sudo plymouth quit

But when i boot, the Ubuntustudio 20.04 show the ubuntustudio splash screen and not my custom plymouth theme (on a live DVD : i'm using cubic to create the custom live DVD)

Am i forgetting some commands ?

I used these commands on Ubuntustudio 19.10 and it works (but not on 20.04):

update-alternatives --install /usr/share/plymouth/themes/default.plymouth default.plymouth /usr/share/plymouth/themes/mytheme/mytheme.plymouth 200
update-alternatives --auto default.plymouth 
update-initramfs -u
update-grub

Answer 1

I found the answer here : https://answers.launchpad.net/cubic/+question/688169

The links (in cubic chroot) /boot were broken. Apply the following command to repair the links (This will create a new option (that you have to select) in cubic available kernels):

apt install --reinstall linux-headers-5.4.0-26 linux-headers-5.4.0-26-lowlatency linux-image-5.4.0-26-lowlatency

then :

update-alternatives --install /usr/share/plymouth/themes/default.plymouth default.plymouth /usr/share/plymouth/themes/mytheme/mytheme.plymouth 200
update-alternatives --auto default.plymouth 
update-initramfs -u
update-grub