It does seem to block some light but I also think it helps diffuse it a little better. However, I'm thinking about trying to put together a custom frame and screen using that clear plastic mesh that has thinner strands that the stock screen. I think the stock screen reflects too much light back up towards my wall and ceiling and it's not all that sturdy. Mine's also old enough that salt corrosion is stating to eat away at the corners of the screen frame.
The space in between the screen and the tank also bothers me. I've lost a few fish because of that 1/4" gap (Yellow Watchman Goby, Engineer Goby, and a Blue Gudgeon Dartfish) and I have to constantly remind myself to check it after I do anything with the lid. I slide it all the way forward now and use the screen holding tabs to block the corners. Since then any fish that have tried to squeeze out have landed in the filtration area in the back and I scoop them out the next day (that's only happened twice though).