High contrast example.

high-contrast.css

body.highContrast {
    background: #000000;
    color: #ffffff;
}
body.highContrast label,
body.highContrast li,
body.highContrast p,
body.highContrast td,
body.highContrast th,
body.highContrast span {
    color: #ffffff;
}
body.highContrast a,
body.highContrast abbr,
body.highContrast acronym,
body.highContrast code,
body.highContrast h2,
body.highContrast h3,
body.highContrast h4,
body.highContrast h5,
body.highContrast h6,
body.highContrast input,
body.highContrast select {
	color: #99ff00;
}
body.highContrast code,
body.highContrast h1,
body.highContrast fieldset,
body.highContrast input,
body.highContrast pre,
body.highContrast select,
body.highContrast td,
body.highContrast th {
	background: #000000 none;
}

high-contrast.html

<a href="#" class="highContrast" title="High constrast">High constrast</a>

high-contrast.js

// Read cookie
function getCookie(strCookie){
    var strName = strCookie + "=";
    var arrCookies = document.cookie.split(';');
    for (var i = 0; i < arrCookies.length; i++) {
        var strValorCookie = arrCookies[i];
        while (strValorCookie.charAt(0) == ' ') {
            strValorCookie = strValorCookie.substring(1, strValorCookie.length);
        }
        if (strValorCookie.indexOf(strName) == 0) {
            return strValorCookie.substring(strName.length, strValorCookie.length);
        }
    }
    return '';
}

// Set cookie
function setCookie(name, value, expires, path, domain, secure) {
    // set time, it's in milliseconds
    var today = new Date();
    today.setTime( today.getTime() );

    /* if the expires variable is set, make the correct expires time, the current script below will set
    it for x number of days, to make it for hours, delete * 24, for minutes, delete * 60 * 24 */
    path = '/';
    if ( expires ) {
        expires = expires * 1000 * 60 * 60 * 24;
    }
    var expires_date = new Date( today.getTime() + (expires) );
    document.cookie = name + "=" +escape( value ) + 
    ( ( expires ) ? ";expires=" + expires_date.toGMTString() : "" ) + 
    ( ( path ) ? ";path=" + path : "" ) + 
    ( ( domain ) ? ";domain=" + domain : "" ) + 
    ( ( secure ) ? ";secure" : "" );
}

// Erase cookie
function eraseCookie(name) {
    setCookie(name, '', -1);
}

$(document).ready(function() {
    var cookieContrast = getCookie("highContrast");
    if (cookieContrast == "highContrast"){
        $("body").addClass("highContrast");
    }
    $("a.highContrast").on('click', function(){
        var x = getCookie("highContrast");
        if (x == "highContrast") {
            $("body").removeClass("highContrast");
            eraseCookie("highContrast");
        } else {
            $("body").addClass("highContrast");
            setCookie("highContrast", "highContrast");
        }
        return (false);
    });
});