Theme switching or Changing Page/Post contrast for readability

Hi,

I thought I might use a theme switching plugin on a site to allow readers to choose a light-text on dark-background style for vision impair readers. I thought I would duplicate the Atahualpa theme and modify the appropriate parts. Of course, I now realize that modifying the duplicate makes the changes appear in both copies of the theme.

Any way around this or is there a better way to provide readers the option of change the background and text colors for readability?

Thanks!
Scott