System UI Tuner not enabling
if you are on marshmallow long press the "GEAR" icon for 5 seconds

   

(you should see a prompt that says ui tuner enabled)


then go to settings and you should see the ui tuner in the settings.


Link with tutorial here http://gadgetguideonline.com/android/android-marshmallow-guide/how-to-use-system-ui-tuner-in-android-marshmallow/