Different themes for Grub 2 boot loader you can find on Gnome-Look website: https://www.gnome-look.org/browse?cat=109&ord=latest
To change Grub them use three simple steps:
1. Copy folder with theme into Grub themes directory:
/boot/grub/themes
2. Edit Grub configuration file /etc/default/grub and write there GRUB_THEME parameter:
GRUB_THEME=/boot/grub/themes/theme_folder/theme.txt
3. Update Grub configuration:
update-grub
Also you can customize GRUB with grub-customizer. Install grub-customizer with command:
sudo apt install grub-customizer
Best three Grub themes:
1. Vimix
2. Tela
3. CyberRe
