1. As root, edit /etc/default/grub and find the line begining GRUB_CMDLINE_LINUX_DEFAULT=
2. Add the commands acpi_backlight=vendor to the end of this line and save the file.
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
Adding acpi_osi=Linux allows the laptop’s function keys to work for backlight control but will likely introduce some power drain issues.
Thus the line would look like:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor acpi_osi=Linux"
3. Run the command update-grub to rebuild the boot configuration.
4. Reboot. You should find you keyboard backlight controls work only when acpi_osi=Linux is included.