Here is 05_debian_theme module

#!/bin/sh -e

. /usr/lib/grub/grub-mkconfig_lib

# this allows desktop-base to override our settings
f=/usr/share/desktop-base/grub_background.sh
if test -e ${f} ; then
. ${f}
else
WALLPAPER=”/usr/share/images/grub/e3grub.tga”
COLOR_NORMAL=”black/black”
COLOR_HIGHLIGHT=”magenta/black”
fi

set_mono_theme()
{
cat <&2
use_bg=true
break
fi
fi
done
break
fi
done

# set the background if possible
if ${use_bg} ; then
prepare_grub_to_access_device `${grub_probe} –target=device ${bg}`
cat << EOF
insmod ${reader}
if background_image `make_system_path_relative_to_its_root ${bg}` ; then
set color_normal=${COLOR_NORMAL}
set color_highlight=${COLOR_HIGHLIGHT}
else
EOF
fi

# otherwise, set a monochromatic theme for Ubuntu
if ${use_bg} ; then
set_mono_theme | sed -e "s/^/ /g"
echo "fi"
else
set_mono_theme
fi