In Firefox: View > Page Style > No Style

It has helped me to read this article. White on black is a terrible choice for long text.

I usually select the text with the mouse to alternate the colors. It's terribly annoying for me, too. I wonder if visual perception differs greatly form one person to the other (I can't explain these design choices otherwise, unless the author has sadistic tendencies)

On a Mac:

System Preferences, search term "Vision". First match should be "white text on black display". Hit enter.