XUbuntu 16.04 Screen Brightness Controls

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

TO

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=Linux"

sudo update-grub 

Run the following command to check the graphics card

ls /sys/class/backlight/

it will list something like following

acpi_video0  acpi_video1  intel_backlight  nv_backlight 

if intel_backlight listed, then it is intel graphic card. while nv_backlight is for nvidia

Now create following file by running the mentioned command

sudo nano /usr/share/X11/xorg.conf.d/20-intel.conf

Add following lines as it is

Section "Device"
        Identifier  "card0"
        Driver      "intel"
        Option      "Backlight""intel_backlight"
        BusID       "PCI:0:2:0"
EndSection

Save the file by pressing "Ctrl O"

Exit the file by pressing "Ctrl X"

Now reboot the machine by running following command 

reboot

It worked for me...hopefully it will do the task for you as well.