Today I was asked to work with a student who is visually impaired and introduce him to Google Apps for Education. When he explained to me that it was easier to view a website with white lettering on a black background, I went to the Chrome store to find a possible extension. I checked the "Accessibilty" category on the left side and found High Contrast. We read the description and reviews, downloaded the extension to Chrome, and opened up several websites. The student was very happy that he was now able to read without struggling. I've put together some instructions if you would like to add High Contrast to Chrome:
I set "Inverted Grayscale" as the default scheme so that my student could go from webpage to webpage without changing the settings. You will also note that you can disable and enable the extension as you desire.