Trying to change the plymouth boot splash screen to solar theme. Go to 'System Settings->Startup and shutdown->Boot Splash' and I select solar theme then apply. It asks for password. But when hitting ok after providing password it shows an error message: 
"Unable to authenticate/execute the action: 2, update-alternatives returned with error condition 2."

The reason for this error is that the solar theme is not "installed" as alternative in the update-alternatives system. So using the update-alternatives --set command (line 69 of helper.cpp) will fail for it. In fact it will fail for every theme that isn't known to update-alternatives. 
This also means if I manually download and extract a theme to plymouths theme directory I am not able to set it

The best solution would be checking with 
update-alternatives --list default.plymouth 
if it outputs the path of the plymouth theme we want to set. 
If its not in there that means the update-alternatives needs to install the .plymouth file first before using set.