The 4 LEDs inside the capacitive buttons use very, very little power. Turning them off won't make a noticable difference in battery life. The millions of LEDs and control circuitry in the screen, that's a different story.
Best phones I've had in 18 years of owning a cell phone ...................
1994 Motorola MicroTAC First "compact" cell phone, ever. My first cell phone, ever.
2007 HTC Advantage First phone with a 5" screen, ever. Had a stylus, too.
2012 Samsung Note Brilliant 5.3" screen. Useable form factor. Decent UI. Stylus. Best phone, ever.