How to make a version of the site for the visually impaired?

Question

css

In general, there is a site, very large and it became necessary to make a version for the visually impaired, in my case it was a change in the color palette ...

The only problem is that the site was not prepared for this. And I was wondering how to prepare for this? And how is this done?

I think that all colors should be set through a class, and in the case of the visually impaired mode, these classes will be interrupted, right?