In order to change Plymouth you firstly need to install Zorin Splash Manager. You can install it from synaptic by searching for splashscreenmanager or by typing:

sudo apt-get install python-gnome2 splashscreenmanager

It will appear in Main Menu→Applications→Preferences→Splash Screen Manager. It will have blue icon.

Firstly you need to download a Plymouth theme. You can search them in the internet, or can download our Plymouth's with Bodhi logo from our #! Plymouth gallery. Keep in mind that YOU DO NOT NEED TO EXTRACT ARCHIVES.
Start application. Then click on button Install, then will pop up windows which says to select your desired splash screen, click OK. Then navigate to file which you downloaded and select it, for example V-Bodhi-spi.tar.gz, click Open, then OK. Window will disappear instantly. Now click Change Default Theme, click OK on window which just showed in. Then you should appear, with your file manager, in place where Plymouth's are stored, select folder (or file inside folder) with same name you previously installed (now it will by V-Bodhi-spi) and click Open and then OK, now wait, it might look that program freezed, but it is not, when job will by done, then new window will appear informing that it is done. Click on it OK and reboot your system.

Sometimes (at random intervals, even before using this software) Plymouth disappears and you get default Plymouth, which is on bottom of screen blue colored line going from left to right.