I personally like command line terminals with a black background and white (or even green) text. However, this is not the most readable color combination and may stress one's eyes after long working sessions. In addition visually impaired people may also like to tweak the screen just for better readability.
There are two posts I've stumple across, which deal with readability. On the Adobe forums one suggested that Adobe Reader may need a feature to invert the page, i.e. to exchange background and foreground, here: The poster complains that reading PDFs is like having a bulb in front of you, which seems a valid point.
Another post comes from the Vinux development blog, here: Vinux is a special linux variant for visually impaired people. One issue for people with bad vision is wrong contrast. Hence, they propose using xcalib for applications like brightness and contrast control using keyboard shortcuts and in addition for an inverted copy of the primary screen on a second monitor, so that one can use the monitor that is more readable.