Have a look at the picture below. This is a keyboard from a V5. I do not know if all the variants in the V5 range have backlit keyboards but I assume if you find the symbol as shown on the 'F9' key then yours does.
If present, you toggle the light on and off with <FN> + <F9> keys.
Please join me in congratulating IronFly who is the first Acer Community Member to achieve the rank of ACE Pathfinder. IronFly has shown a consistent willingness to help others and has worked hard to earn this new Rank. Congrats and Thank You IronFly.