Two eaters, one non-eater
Well, I don't use lighting in my cages, and my room light is usually off during the day; so they get just the normal faint daytime light that would shine through the blinds that are closed..lol. They seem to be doing just fine with no lamps or lighting. Hmm... What setting is your heating pad on? (low, medium, or high?) I would try to see if you can bump it up alittle bit, and take the lamp out, see if that would work. I could almost guarantee you that the humidity would go up a few.
As for the screen top, I've heard of wrapping duct tape around the top of it to keep humidity in. I personally have never done it. What I do for my 55 gallon tank is use pegboard as a lid. It works GREAT for keeping the humidity in. A big 2x4 sheet of it is like $5 at home depot... Then just cut it accordingly. Screen tops are seriously the biggest pain for keeping humidity lovers.
Jennifer