Anything you put between the light and the water will reduce the light some.. In general regular glass will allow roughly 90% of the light to pass through it (acrylic is usually ~92-93% of the light passes through it)..
And as hinted at "gas exchange" can be quite critical in an aquarium.. In a natural denitrification process.... ammonia gets converted to to nitrites to nitrates which is then broken down into essentially nitrogen gas and will bubble up and out of the water.. If you stop that you are essentially stopping complete export.. The water also needs to be oxygenated..which powerheads/skimmers,etc.. do
So don't fully seal the tank up but leaving some place for oxygen/nitrogen,etc.. to be exchanged from the tank into the rooms air,etc.. is important..
So.. use either glass or acrylic for the top but don't fully seal it..
If you have a sump or skimmer then you are better off as that will help oxygenate the water,etc.. too..
Personally IMO its best to not cover a tank and to simply deal with the evaporation.. Its what it is.. reducing it some is ok via a top but you won't stop it all and it actually provides cooling (evaporation helps to keep the water cool and remove heat from pumps,etc..) as well as gas exchange..