The System UI Tuner in Android Marshmallow is a hidden menu and by activating it you get to edit the status bar, quick settings and some more. To turn on System UI Tuner in Android 6.0 Marshmallow, do as follows:
1. Drag down the status bar
2. Tap and hold your finger on the Settings icon at the top right
3. Keep holding for about 5 seconds until you see this popup message on the screen: “Congrats! System UI Tuner has been added to Settings.”
4. Now open the Settings application, scroll all the way down and you should see the newly added menu: System UI Tuner.