Remove splash screen at login

Lang: bash 
echo 'GRUB_CMDLINE_LINUX_DEFAULT="quiet"' >> /etc/default/grub

Add splash option to enable Plymouth

Lang: bash 
cat <<EOF >> /etc/default/grub
if ! echo "$GRUB_CMDLINE_LINUX_DEFAULT" | grep -q splash; then
    GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT splash"
fi
EOF

Setup grub theme

Force a 16x9 mode first, then 16x10, then default for grub theme.

Lang: bash 
cat <<EOF >> /etc/default/grub.d/kali-themes.cfg
GRUB_GFXMODE="1280x720,1280x800,auto"
GRUB_THEME="/boot/grub/themes/kali/theme.txt"
EOF

Change grub background

Replace the background image mentioned in kali/theme.txt i.e. /boot/grub/themes/kali/background.png.