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I don't think an xorg.conf will make `xbacklight` work (but I may be wrong).
However, I do know that my AMD laptop (with the free radeon drivers) allows for keyboard backlight control if xfce4-power-manager is installed and running; the GNOME desktop also supports the backlight keys without needing that program.
Alternatively, you could try fiddling with the kernel parameters to see if that helps:
"nothing worked" is not a valid statement on a technical forum.
but...
if X isn't running, xbacklight won't work. simple.
apart from that, i read multiple statements (and my own experience) that xbacklight simply doesn't work.
try compiling https://github.com/haikarainen/light - it's easy, and has worked on my system, where xbacklight failed.