# the below not found on Ubunta Mate 18.04 - present on Arch?
plymouth-set-default-theme YOURTHEME -R

update-alternatives --install /usr/share/plymouth/themes/default.plymouth default.plymouth /usr/share/plymouth/themes/"path/to-your-plymouth.plymouth" 100
e.g.
update-alternatives --install /usr/share/plymouth/themes/default.plymouth default.plymouth /usr/share/plymouth/themes/angular/angular.plymouth 100

update-initramfs -u
# -u update an existing initramfs