Probably what you are thinking of is the jqueryui skin in our extra-skins download, it has a switcher for the jqueryui theme. 

We don't have something like that for switching skins for anonymous users.

In the old days some sites used to add a link to apply a different css for larger font or better contrast, but that is no longer in common use since all modern web browsers have built in functionality for increasing font size or magnifying/zooming. Generally it is best to make your skin have good contrast for everyone instead of having to click something for it.