Thank DdB! i have some success
/usr/share/desktop-base/homeworld-theme/grub/
boot screen can be changed by replacing file in location above
i haven't been able to change login background
Very strange: if i use my favorite search engine, i find dozens of
places, describing a solution. And especially nowadays, where AI
specializes on giving useful hints and explanations to remedy computer
configuration problems, i am wondering as to why you keep asking here?
I have neither lxde nor lightdm, so cannot even verify those solutions,
but you should ... beginning with perhaps:
https://www.marksayson.com/blog/lxde-customizing-the-log-in-background/
.