The solution is invariably a simple command pushed to grub in order get your bios to play nice with the gui & Fn buttons, but the exact command depends on your hardware. If you've got Intel graphics, a Lenovo, or the like, try the following, which works 99% of the time when 'acpi_osi=Linux' does not:
Open Terminal,
sudo echo 'acpi_backlight=none' | sudo tee /etc/kernel/cmdline
sudo clr-boot-manager update
Reboot.
