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.