[knolax]

I have astigmatism and I have no idea what the author's talking about. I stare at white on black terminal text all day without issue (maybe it's different for presentations?). I've also made presentations with white on black text before and had no problem reading the slides.

[nullc]

If the overall environment is really dim, your pupils will dilate. Like opening the aperture of a camera this will make the area of correct focus smaller.

... but if the room is so dark that the presentation background alone is all that is keeping my eyes from dialating, then I'm going to find the presentation uncomfortably bright.