NOTE: I am not after themes… more just hacking what's here as its faster.
In Ubuntu 18.04 I am trying to change the
grub.png/splash.png image, the one you see when GRUB asks you what system you want to boot up, Linux/Windows/etc.
I know how to give myself permissions over a whole folder and its files in
usr/share/background/ (which is how I changed my login screen), but if I am going any place near GRUB I want to only give permissions to the actual image – not to GRUB or kernel or other files inside.