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.
Our friend and comrade IronFly has reached his 10,000th post to our little Community. During the last 4 years he has helped countless Acer customers providing help, advice and answers. The Acer Community team feels honored to count you as our friend. Thanks IronFly!