In Ubuntu 14.04, I was able to change my Plymouth Theme simply by going to /lib/plymouth/themes and copy my theme in there.
Then, by simply typing sudo update-alternatives --config default.plymouth (choosing the accordingly number of my theme) and sudo update-initramfs -u, I was able to change my Plymouth theme.
In Ubuntu 16.04 LTS, the folder /lib/plymouth/themes does not even exist.
By seraching in Nautilus, I was able to find the folder /usr/share/plymouth and place my theme under themes. Then, by typing in the shell the commands above, I was, theorically, able to change my Plymouth theme to my pre-copied one.
The truth is that, after changing the Plymouth theme to my own and after several reboots, no Plymouth theme is showed at boot. Just a black screen appears and then lightdm shows up and I login to my account…
I'm using burg as my default bootloader.
Best Answer
I ran into a similar thing. After using synaptic to find the default plymouth theme, I hit properties to see what files were installed. That listed most files going into this directory
/usr/share/plymouth/themesput all you files there then run