In their infinite wisdom, most US carriers opted to remove the Galaxy S6 settings to change the backlight timeout for the capacitive back and overview buttons (AT&T didn't for some reason). This may seem like a minor point, but it's actually rather annoying. Now there's an app that can fix that, and it's free.
The system hooks for changing the backlight timeout are still there, and you can access them via ADB commands. However, Galaxy Button Lights can do that for you in app form, and it doesn't need root. Simply choose a time with the slider and hit the save button. Alternatively, you can set them to always on or off. I've tested this, and it works wonderfully.
So why would you want to do this? The GS6's screen is great for use in a dark room as it gets super-dim without distorting, but the button backlight ruins it. It's blazing bright by comparison and stays on for several seconds each time you touch the screen. Changing the timeout or disabling the backlight is sometimes necessary.