Add a GRUB theme

Download a theme from https://www.gnome-look.org/browse?cat=109&ord=latest

How to install a grub theme:

1. Extract the theme to /boot/grub/themes/
2. Edit /etc/default/grub file. Find the line starting with “#GRUB_THEME” and change it to “GRUB_THEME=/boot/grub/themes/[THEME_FOLDER]/[theme.txt]” (replace [THEME_FOLDER] ; [theme.txt] is probably OK ; remove “[]” brackets)
3. Run sudo update-grub