Do not worry about it cutting off the oxygen supply to the tank, because no cover fits so tight that it will keep out the necessary amount of air. Finding the correct location for the tank is also very important. An aquarium should not stand in a drafty spot or near a radiator. Sudden changes in temperature can be bad for your fish. Direct sunlight should be avoided except for maybe short periods of time.
Artificial lighting is just a good for the fish as daylight.
To mush light will encourage the growth of algae, which will turn the water green. Too much direct sunlight can also overheat the water. The gravel, water, and plants put in your tank are all very important to the health of the fish and to the looks of your aquarium. Gravel or sand is not necessary except if you are using rooted plants. It does make for a more natural looking setting for the fish.