Here are two ways to turn off the button backlight
1. Install "Darker". Enable "Dim capacitive button lights". You can also tick "start on boot" in settings.
2. Xposed framework and Gravitybox (Recommend)
Flash xposed-arm-20150213b.zip via recovery
Install XposedInstaller_3.0-alpha1.apk
Search "gravitybox" in Download. Install and activate it, reboot
Go "Display tweaks" in Gravitybox. Change button backlight mode to disable.