KonkyBonky This is a kernel issue. The kernel is suppose to trap these WMI keys and change the brightness on its own, and if it doesn't, pass the key up to userspace (like the NetBSD kernel does) so that something can be done there.
xev -event keyboard and find out what
keysyms are being sent. These should look like
XF86...BrightnessDown. Then, you can trap those keysyms either in the WM, or using a program like
xbindkeysand run commands to change the brightness.