to install :
- copy the content of this folder to /boot/grub2/themes/koozali
- add to /etc/default/grub:
GRUB_THEME="/boot/grub2/themes/koozali/theme.txt"
GRUB_GFXMODE="1024x768"

- then execute :
# grub2-mkconfig  -o /boot/grub2/grub.cfg