Plymouth is responsible for the boot splash and login screen in Ubuntu 10.04 onwards, having replaced Usplash/GDM in earlier versions (and the short-lived use of XSplash in 9.10). There are a bunch of themes available in the repositories, and you would expect all to go well if installing them via Synaptic, but it could end in disaster.
My personal experience with changing themes was the inability to boot back into Ubuntu the very first time I did it, but I’m pretty sure I found the reason why: an incomplete theme. The splash did display at boot, along with its animation, but after that the screen went blank, and all hard drive activity stopped.
Now, let me point out the theme in question (Spinfinity) is not only one of the most popular ones out there, but is in the official repos, and easily installed via Synaptic. However, looking in its folder in /lib/plymouth/themes, I could see one major difference between its contents and those of of the official Ubuntu, Kubuntu and Xubuntu ones (which I knew work): there is no .script file.
For example, the default Ubuntu theme has a lengthy script called ubuntu-logo.script, but for some reason the .script file is missing in the theme that caused the boot failure.
So, if you want to make sure an newly installed theme is not going to cause major headaches, go to its folder in /lib/plymouth/themes and check there is a .script file (with the name of the theme as the prefix) in there.
What to do if you can’t boot into Ubuntu? Reboot in recovery mode, enter your username and password at the command prompt, and then startx to get to your desktop. Then just switch back to a theme you know works, like the default Ubuntu splash, and you should be OK after that.
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