Continuing the discussion from Lenovo Thinkpad and EOS:
i noticed brightness keys were working in live environment but not working post install after a system update.
on checking keys settings i figured out brightness keys are not handled by xfce power manager.
so what i did is goto settings editor --> xfce4-power-manager–>handle-brightness keys
and tick the value.
Fn+F8/F9 is able to decrease or increase the brightness now!