Here is how to get the Fn F6 and Fn F5 keys working on a brand new 2017 Thinkpad X1 Carbon.
The keys are already correctly mapped to, respectively, XF86MonBrightnessUp and XF86MonBrightnessDown, but there is nothing assigned to these keys. Try this:
A create a /etx/X11/xorg.conf with
Section "Device" Identifier "Card0" Driver "intel" Option "Backlight" "intel_backlight" EndSection
See Debian bug #651741 for some more information.
B in ~/.i3/conf add:
bindsym XF86MonBrightnessDown exec "xbacklight -dec 10" bindsym XF86MonBrightnessUp exec "xbacklight -inc 10"
See pastebin code
C Restart i3 with Mod R, or restart X.