Text is much easier to read when there is a sufficient contrast between the text and the background. Black text on a white background is the de facto standard for both print and the web. However, this combination is not ideal for all users. Users with very low vision may set the background to black and the text to white or yellow. Users with dyslexia may set the background to an off-white color or light yellow, with black text. Some people with dyslexia lay a clear sheet of tinted plastic over the screen in order to read more effectively. Web developers cannot control for these user behaviors, and they do not have to. Users will do what they need to do, and what they are accustomed to doing, in order to read. The main concern for web developers is to ensure a high degree of contrast for the general population of readers.
Tools such as WebAIM's Color Contrast Checker make it easy to check contrast thresholds and determine WCAG compliance.