I agree, it is very subjective to decide what is easy on the eyes. Some people like to watch TV in a completely dark room, others not.
My suggestion was to give the user the choice how a PDF should be displayed. If you want to maximize contrast, you may read it in yellow on black background. The black font on light gray background in my first post was just one example of many possible variations.
Thank you again for considering my feature request.
In Acrobat Reader, it’s possible to change background and text color (under Preferences → Accessibility).
The request is noted, but always be aware of what Adobe does. They do a great many things with PDFs and PDF presentation that are specific to them. This does not mean these options are available to or easily implemented by other developers. Look at other PDF applications, like Preview and Skim, and you’ll notice they don’t offer the option you mention.