For this diff, test case is rather simple. Just install lubuntu-grub-theme and make changes to /etc/default/grub as shown in summary. Run grub-update. That's it.
Important part lies after this diff lands.
lubuntu-grub-theme should be added to rSEED and rLUBUNTUMETAPACKAGING.
Once both of them are landed,
The new iso created should install the theme by default during installation of grub.
Now, Ideally it shouldn't. (According to grub's manual) But looking at theme from mate, I can say it will.
Spoiler alert: If it does, then we have a bug in grub.