My current setup comprises Fedora with the i3 window manager running atop Gnome on my Thinkpad. Sadly, the recent transition to FC25 broke brightness control with xbacklight, resulting in lots of tinkering with xorg.conf and dbus to no avail.

In the end, I gave myself sudo access to /sys/class/backlight/intel_blacklight/brightness without password prompting because I want to remain somewhat sane (who wants to enter passwords after every brightness change?).

Next, I created the following script to adjust the brightness accordingly:

#!/bin/bash

path='/sys/class/backlight/intel_backlight/'
file='brightness'
now=`cat $path$file`
max=`cat $path'max_brightness'`

if [ $1 == "i" ] && [ $now -lt $max ]
then
    new=$((now+$2))
elif [ $1 == "d" ]
then    
    new=$((now-$2))
fi

echo $new | sudo tee $path$file

And bind as follows in i3.config:

bindsym XF86MonBrightnessUp exec <path> i 35     # Increment
bindsym XF86MonBrightnessDown exec <path> d 35   # Decrement

Reload and, hey presto, problem solved.