making site friendly for people with visibility issues

I'm building a website with a blue color scheme. It looks nice. But one person is complaining that they can't see it very well because of the contrast, and want to change everything. I've put a lot of work into it already and do not want to change it all just to satisfy a minority of people. But I do want to help them use the site easier.
Is there some kind of button I could add to the page which if clicked would change the color scheme to a more basic black text on white background? But keep everything else the same?

Thanks

 

of course. You can just have a button set a cookie, and your code check for the cookie before it assigns the style class to your object.

 

Great - thanks. If you dont mind me asking - how would I do that exactly?