Rosalind - 11:55 am on Dec 20, 2012 (gmt 0)
Making a copy of the entire site is pointless, and bad for SEO. What you need is a stylesheet switcher. You'll find loads of code examples online. The advantage over jecasc's solution is, apart from not demanding that your visitors think for themselves, you can add high-contrast styles, fonts that are chosen for their legibility, and a design that's optimised for someone with poor fine motor control.