First, download themes. Search ‘grub’ on gnome-look.org, you can find many themes for grub
some of them are .deb files which you can install using this command:
dpkg -i path-to-downloaded-theme
and some of them have an installation script If it gives ainstall.shin the theme package, you can easily install the theme by running the script via root privilege. In terminal, navigate to extracted theme folder and execute:
sudo ./install.sh
Or, you can manually install the theme. Put the theme folder under /boot/grub/themes(create the themes folder if not exist).
Then edit “/etc/default/grub” file, add this line:
GRUB_THEME=/boot/grub/themes/theme-folder/theme.txt #PATH_TO_THEME.TXT
Finally, update grub to take effect by command:
sudo update-grub

hint: if you want the theme in the picture above contact me and I'll get it to you