The function keys don't work for me either. The hardware either isn't supported (ACPI) or the keys aren't. I have been able to dim it properly though through KDE system settings. I think under power or something.
Samsung Galaxy Precedent (rooted, AreaRom 3.4 , sd-ext w/ 3.65GB internal space)
Kindle Fire (1st Gen, rooted, AOKP JB 4.1, Nova Launcher)
Samsung Galaxy SIII 4G LTE (rooted, Carbon ROM)