From MEPIS Documentation Wiki
Plymouth is a graphical boot splash screen that supports animations. It makes use of the Direct Rendering Manager (DRM) and kernel-based mode-setting (KMS), as well as the framebuffer driver for non-KMS drivers (requires the plymouth-x11 package). It resides in initrd.
Plymouth is in the repos, and can be installed in the usual manner.
Plymouth comes with a few basic themes, and others are availble from the repos. MEPIS 11 installs with a default MEPIS theme named MEPIS-11.0.
To change themes, issue this command as root:
/usr/sbin/plymouth-set-default-theme -R <nameoftheme>
This sets the default theme in /etc/plymouth/plymouthd.conf, and then rebuilds the initrd file by calling "update-initramfs -u"
Tips and tricks
Most plymouth themes on the net are intended for Ubuntu, which has its themes in /lib/plymouth/themes instead of /usr/share/plymouth/themes. As well as installing them into the correct location, the configuration .plymouth files must be edited to use the MEPIS location.