To change between the installed themes you should do:

sudo update-alternatives --config default.plymouth
sudo update-initramfs -u

The first will list available themes to choose from, the second command with update initramfs with a new theme.

Getting Started Guide

Editing Plymouth

You can edit Plymouth with a simple text editor for animations and images. This is Ubuntu version agnostic.

• Get Template
• Customize Template
• Set Custom Theme

Make a template

First you will get a template that covers all the basic aspects of a Plymouth. Change my-theme to your own personal them name.

sudo cp --recursive /lib/plymouth/themes/ubuntu-logo ~/themes/my-theme

Customize file names

Now you can change some names to help identify our theme from others.

sudo mv ubuntu-logo.plymouth my-theme.plymouth
sudo mv ubuntu-logo.script my-theme.script
sudo mv ubuntu-logo.grub my-theme.grub
sudo mv ubuntu_logo16.png my_theme16.png
sudo mv ubuntu_logo.png my_theme.png

Edit image files

Use your image editor of choice to alter the png images. I like GIMP for quick edits.

gimp my_theme.png my_theme16.png 
gimp progress_dot_on.png progress_dot_off.png 
gimp progress_dot_on16.png progress_dot_off16.png

Here is an example of my alterations:

Edit the configuration files.

1. Set up theme information file.

   gedit my-theme.plymouth
   

   Edit lines 2, 7, and 8 and change ubuntu logo to you theme name.

   [Plymouth Theme]
   Name=My Theme
   Description=A theme that features a blank background with a logo.
   ModuleName=script
   
   [script]
   ImageDir=/lib/plymouth/themes/my-theme
   ScriptFile=/lib/plymouth/themes/my-theme/my-theme.script
   

2. Start-up terminal color. (Optional)

   gedit my-theme.grub
   

   Change it from purple to something else.

   Edit line 1 with RGB color code.

   A good site to help you is Color-Hex. I am just going to make mine black.

   if background_color 0,0,0; then
     clear
   fi
   

3. The script file to apply theme changes

   gedit my-theme.script
   

   There is many different aspects to this file that you can change to customize your plymouth boot theme. If you wish to do more advanced changes you should study it. I am only showing how to change the background color, logo, and status indicator.

   Edit lines 169, 170, 174, and 180 to change the background gradient and images you edited earlier.

   Window.SetBackgroundTopColor (0.66, 0.66, 0.66);       #top 
   Window.SetBackgroundBottomColor (0.146, 0.146, 0.146); #bottom
   
   bits_per_pixel = Window.GetBitsPerPixel ();
   if (bits_per_pixel == 4) {
       logo_filename = "my_theme16.png";
       progress_dot_off_filename = "progress_dot_off16.png";
       progress_dot_on_filename = "progress_dot_on16.png";
       password_field_filename = "password_field16.png";
       question_field_filename = "password_field16.png";
   } else {
       logo_filename = "my_theme.png";
       progress_dot_off_filename = "progress_dot_off.png";
       progress_dot_on_filename = "progress_dot_on.png";
       password_field_filename = "password_field.png";
       question_field_filename = "password_field.png";
   }
   

Send your theme folder

cp --recursive ~/themes/my-theme /lib/plymouth/themes

Set Plymouth configuration to use theme.

sudo ln -sf /lib/plymouth/themes/my-theme/my-theme.plymouth /etc/alternatives/default.plymouth
sudo ln -sf /lib/plymouth/themes/my-theme/my-theme.grub /etc/alternatives/default.plymouth.grub

18.04+

sudo update-initramfs -u

Restart to watch it in action.

You can get a list of themes available via the repositories through Synaptic Package Manager or through the command line via aptitude:

$ aptitude search plymouth-theme
p   lubuntu-plymouth-theme        - plymouth theme for Lubuntu
p   plymouth-theme-fade-in        - graphical boot animation and logger - fade-in theme
p   plymouth-theme-glow           - graphical boot animation and logger - glow theme
i   plymouth-theme-kubuntu-logo   - graphical boot animation and logger - kubuntu-logo theme
p   plymouth-theme-sabily         - plymouth theme for Sabily
p   plymouth-theme-script         - graphical boot animation and logger - script theme
p   plymouth-theme-solar          - graphical boot animation and logger - solar theme
p   plymouth-theme-spinfinity     - graphical boot animation and logger - spinfinity theme
p   plymouth-theme-text           - graphical boot animation and logger - text theme
c   plymouth-theme-ubuntu-logo    - graphical boot animation and logger - ubuntu-logo theme
i   plymouth-theme-ubuntu-text    - graphical boot animation and logger - ubuntu-logo theme
p   plymouth-theme-ubuntustudio   - Ubuntu Studio Plymouth theme
p   xubuntu-plymouth-theme        - Plymouth theme for Xubuntu

You can then install the resulting packages via Synaptic or apt as normal:

$ sudo apt install plymouth-theme-solar

If you're looking for something not in the repositories, UbuntuGeek has an excellent tutorial on how to install and create your own custom Plymouth themes.

I ran into the same problem, and I came here. No luck. I started randomly browsing around the Plymouth framework, and I noticed the default.plymouth file was a linked file. Every time I set the splash screen with the terminal, the file updates. Through trial and error, I found this method.

In the terminal, type in the following: gksu [file_explorer_name]

This will open a new window, with root (superuser) permissions to edit, delete, and add files. Navigate to /usr/share/plymouth/themes/. This is where your themes are located. If your theme folder isn't in the plymouth themes folder, move it. In your themes folder, find a .plymouth file, and open it in your default text editor. Look at the directories, and make sure they're accurate and up-to-date. Save and exit the text editor. Make a LINKED file of your .plymouth file, and drag it to the themes folder (/usr/share/plymouth/themes/) and rename it as default.plymouth You need to delete the existing files (if any) in the themes folder. After that, you should be able to close the window and terminal, then reboot to test the configuration.