Mac OSX offers an easy solution to increasing the contrast of your display, and switching the screen from black on white to white on black. In the Universal Access section of System Preferences, under the large "Display" label, you will find a set of controls. Switching from black on white to white on black will invert the colors on the screen, making it easier to read for some people. There is a toggle to the right of that control which switches your display from color to grayscale. Below that, a slider allows you to increase the contrast of your display for increased text readability.