1) You can modify /etc/default/grub uncommenting GRUB_THEME="/boot/grub/themes/breeze/theme.txt"
2) Then you can modify /boot/grub/themes/breeze/theme.txt: for example you can
- add a background: adding, in # General settings, a line such as "desktop-image: "your-image-in-the-same-folder.png" (obviously you have to put, as root, that image into the folder);
- reducing the height for menu items, in "+ boot_menu": "item_height = 15" (not: 33)
fine tuninga) you can change the screen resolution (in /etc/default/grub);
- give this command "xrandr" in order to know the supported resolutions
- then uncomment/modify GRUB_GFXMODE="your desired resolution"
for example "sudo grub-mkfont --output=/boot/grub/fonts/DejaVuSans-10.pf2 --size=10 /usr/share/fonts/truetype/dejavu/DejaVuSans.ttf"
c) per installare nell'mbr
sudo grub-install /dev/sda