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 Note 3 (rooted, HyperDrive ROM)
Samsung Galaxy Tab 3 7.0 (non-rooted, stock)
Samsung Galaxy Tab 2 10.1 (obsolete, slow, rooted)
Samsung Galaxy Note 10.1 2014 Edition