edit grub background and timeout , default os ubuntu 20.04
change grub image background and the timeout without extra software on ubuntu 20.04 :
-copy your image background to : /boot/grub
-edit timeout seeting: sudo nano /etc/default/grub
-edit your os nummber
after edit :
ctr + O
entry
ctr + X
-update your grub: sudo update-grub