You could try searching for Admin themes but I doubt there'll be much to help this particular user out of the box. I'd suggest grabbing the most minimalistic Admin theme you can find and then hacking it.
Yellow text on a black background with green links works well if you need a high contrast display and, if possible, reverse the colors for the onhover, onactive and onfocus states for all links and buttons. I'd also look at getting rid of any button background images and setting button font sizes in ems. In fact, try to change all font-size specs to ems.
If you need a hand with this, give me a shout via esmi at quirm dot net. I've got some experience creating displays for users with different needs - including visual impairment. I'd also like to see a good, high contrast Admin theme made available on wordpress.org, if possible.