How to create an optionally available stylesheet for visually impared users?

Easy to setup a button trigger/toggle with a bit of jQuery. Here’s a working example I made for you:
https://jsfiddle.net/8Le2tjvr/1/

The Code:

HTML:

<button id="impaired-button">Impaired Button</button>

jQuery

$('#impaired-button').click(function() {
    $('body').toggleClass('impaired');
});

CSS

.impaired h1 {
    font-size: 50px; 
}

Just add .impaired before any css that you want specifically styled for impaired users. This will be shown when the button is clicked.