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

# copy that file e.g. to this path
/usr/share/images/background-grub-ideal-linux.png

vim /etc/default/grub; # at the very end of that file add

GRUB_BACKGROUND="/usr/share/images/background-grub-ideal-linux.png"

# quit, save and
update-grub; # reboot and checkout if it works :)

here is an example file:

change colors:

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)

        if [ -z "${2}" ] && [ -z "${3}" ]; then
                echo "  true"
        fi

and, replace them with the following:

if [ -z "${2}" ] && [ -z "${3}" ]; then
    # echo "  true"
    echo "    set color_highlight=red/yellow"
    echo "    set color_normal=white/black"
fi
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