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.
Member RevoMaxx is quickly moving up the ranks of Acer Community. He joined us in September and is already making a difference. It's great to see new members like him make a difference only after a few months. Thanks RevoMaxx!