tested on Debian 8.7
change background picture:
use gimp to create a png with the dimensions: 640px x 480px
(to reduce filesize go Image -> Mode -> Indexed -> 256 Colors… )
Ctrl+E -> Export to background-grub-ideal-linux.png
vim /etc/default/grub; # at the very end of that file add
# quit, save and
update-grub; # reboot and checkout if it works :)
Now to change the colors, open vim /etc/grub.d/05_debian_theme and find the following line: (debian 8.7 it used to be around line number 124)
echo " true"
and, replace them with the following:
# echo " true"
echo " set color_highlight=red/yellow"
echo " set color_normal=white/black"