I installed xbacklight, hoping that it would be able to control my brightness. When trying to change brightness, I was told "No outputs have backlight property". After looking around, I found that this has to do with my computer not having a /sys/class/backlight folder. I know that this is the problem but don't know exactly how to fix it.

Not sure if its needed or not but here some system info from inxi:

System:   Kernel: 3.16.0-57-generic x86_64 (64 bit, gcc: 4.8.2) 

          Desktop: LXDE (Openbox 3.5.2) Distro: Ubuntu 14.04 trusty

Machine:  HP EliteBook 8460p

CPU:      Dual core Intel Core i5-2520M CPU

Graphics: Advanced Micro Devices [AMD/ATI] Seymour [Radeon HD 6400M/7400M Series] 

Does this have to do with drivers (perhaps Intel/AMD graphics drivers)?

That is completely normal. To find the directory for your backlight settings, do this:

sudo find /sys/ -type f -iname '*brightness*'

The output should give you something like this:

/sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight/brightness

Now, all you have to do is link it to /sys/class/backlight. To do that:

sudo ln -s /sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight  /sys/class/backlight

If you still get the error, then do this:

Create the file xorg.conf:

sudo nano /etc/X11/xorg.conf

And add these lines:

Section "Device"

Identifier  "Card0"

Driver      "intel"

Option      "Backlight"  "NAME OF THE FOLDER"

EndSection

Then, to save the file do: Ctrl + X then Y then Enter.

Also, for the Driver part check for your configuration, i.e., acpi_video0 or intel_backlight.

Check your /sys/class/backlight folder. If you can see an intel_backlight folder there and still you are getting the above error then creating a /etc/X11/xorg.conf file with the below configuration should work for you. It worked for me.

  Section "Device"

        Identifier  "Intel Graphics" 

        Driver      "intel"

        Option      "Backlight"  "intel_backlight"

    EndSection

Also, remember to logout and login again for the changes to take effect.

Reference:
https://wiki.archlinux.org/index.php/backlight

Every step required for xbacklight control

Follow these steps:

1. 
   $ sudo nano /etc/default/grub and replace the corresponding line with GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
   


2. $ sudo update-grub


3. No joking, make sure that the appropriate drivers are actually installed: $ sudo apt install xbacklight xorg xserver-xorg-video-intel
   


4. Issuing $ find /sys -type f -name brightness should yield something like /sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight/brightness

   


5. 
   $ cd /sys/class This directory should contain a soft link called brightness to the brightness device discovered in the previous step. Should it be missing, create it: $ sudo ln -s /sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight/brightness /sys/class/brightness
   


6. 
   $ sudo nano /etc/X11/xorg.conf should read:

Section "Device"

    Identifier      "Device0"

    Driver          "intel"

    Option          "Backlight"      "intel_backlight"

EndSection



Section "Monitor"

    Identifier      "Monitor0"

EndSection



Section "Screen"

    Identifier      "Screen0"

    Monitor         "Monitor0"

    Device          "Device0"

EndSection

7. The assignment of the physical XF86MonBrightnessDown and XF86MonBrightnessUp keys is explained here for Xubuntu LTS or XFCE users.


8. Finally, reboot for these changes to take effect.