Grub: Grub is a type of boot loader used to boot into main OS.
Updating Grub Background
Step 1: Follow /boot/grub/themes/kali/ directory
Step 2: Open “theme.txt” with text editor. (with sudo permissions)
Step 3: Replace desktop-image: “background.png with your image name, and save file. (make sure to copy image in same /boot/grub/theme/kali/ before modifying “theme.txt” file)