To install:

--Copy zorin folder to /lib/plymouth/themes

Type to terminal:

sudo update-alternatives --install /lib/plymouth/themes/default.plymouth default.plymouth /lib/plymouth/themes/zorin/zorin.plymouth 100

sudo update-alternatives --config default.plymouth

select the number corresponding to zorin then run

sudo update-initramfs -u


To fix the delayed loading of the splash:

sudo -s
echo FRAMEBUFFER=y >>/etc/initramfs-tools/conf.d/splash
update-initramfs -u

for more details visit http://community.linuxmint.com/tutorial/view/1149

to use the grub background install grub customizer following this http://www.noobslab.com/2013/04/customize-grub-with-grub-customizer-in.html