I don't think it happens to everybody with astigmatism. I cannot read any text with black background. After a very short amount of time I'm unable to read anything, and once I take my eyes off the screen it feels as though I've been staring into the sun.

There used to be a great Firefox plugin that allowed me to change the colours of web pages that use black background, but it doesn't work anymore, and I haven't found a good replacement.

This has to do with screen glare. A matte screen might treat you better.

I only use matte screens. It definitely isn't that though. I didn't mean to say that black screens are too bright, that wouldn't make any sense.

What I tried to explain was the sensation, not the actual effect. It was a really bad example, I agree.