I'm running Linux Mint 18 Cinnamon on my Asus X555LA, which also has Windows 10 installed on another partition (the backlight works perfectly in Windows).However in Linux Mint: there's already an issue with Fn+F5/F6 not changing the backlight intensity. The various fixes I've tried (but haven't worked) include:adding a 20-intel.conf file into the /usr/share/X11 folder.added acpi_osi= to the /etc/default/grubRunning acpi_listen and pressing Fn+F5/F6 shows no output.Installing xbacklight and running xbacklight -set $percentageUntil about two days ago I could skirt this issue by using the brightness slider in the UI, however even this no longer works and the screen brightness is stuck at 100%.I am at a loss and no longer know what to do. 
Backlight slider no longer works in Linux Mint
linux mint;acpi;asus;backlight