I have done this in the past in 16.04 and 17.10 but in 18.04 I can not get plymouth manager to work. Downloaded it from the software center but it doesn't work. Is there a guide available that will let me do it in terminal?
The same methods described here, do still work in 18.04.
NB: between 14.04 and 16.04 the location of Plymouth themes changed to
/usr/share/plymouth/themes. If you have an old theme its
.plymouth file might point to
Installing a new theme
Easy if it's packaged -
sudo apt install
Otherwise you need to copy its folder or extract its archive to
/usr/share/plymouth/themes. Then you need to
sudo update-alternatives --install /usr/share/plymouth/themes/default.plymouth default.plymouth /usr/share/plymouth/themes/THEME/THEME.plymouth 100
...replacing THEME with the theme name.
Choosing the theme
sudo update-alternatives --config default.plymouth sudo update-initramfs -u
You can also test Plymouth without rebooting...
sudo apt install plymouth-x11 sudo plymouthd ; sudo plymouth --show-splash ; for ((I=0; I<10; I++)); do sleep 1 ; sudo plymouth --update=test$I ; done ; sudo plymouth --quit