For this tutorial I will be using this debain theme

https://www.gnome-look.org/p/1368573/

To begin with, download the .zip file and extract its contents to

usr/share/plymouth/themes

Now issue these commands on terminal.

sudo update-alternatives --install /usr/share/plymouth/themes/default.plymouth default.plymouth /usr/share/plymouth/themes/debian/debian.plymouth 100

sudo update-alternatives --config default.plymouth

Now type in the selection number that corresponds to the theme you downloaded. Here I will choose “2” for debian.plymouth

Lastly, issue this last command to update everything and Reboot.
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