Hello!
I believe plymouth controls the LL boot screen, so GRUB Customizer won't help you with THAT. However, it WILL let you add a pretty picture to your GRUB bootloader. For some examples, grab grub2-splashimages:
sudo apt-get install grub2-splashimages
You will then find these images in the /usr/share/images/grub folder.
Here's a link to a way to change the plymouth splash screen, but personally, I don't recommend 'playing around' with plymouth, as one boo-boo can leave your LL system unbootable, and I don't think Boot Repair can fix such a mistake.
There is a GUI called Plymouth Manager, but I DEFINITELY don't recommend it - again, for the reasons above.
Oh, yeah, the plymouth link:
http://theurbanpenguin.com/wp/index.php/ubuntu-14-04-custom-boot-screen/Best of luck...
73 DE N4RPS
Rob