Edit in JSFiddle

var WebFontConfig = {
    google: {
        families: ['Pontano Sans']
    },
    timeout: 3000
};

(function () {
    var wf = document.createElement("script");
    wf.src = ('https:' == document.location.protocol ? 'https' : 'http') +
        '://ajax.googleapis.com/ajax/libs/webfont/1/webfont.js';
    wf.async = 'true';
    document.head.appendChild(wf);
})();

document.getElementById('switch-button').addEventListener('click', function () {
    if (document.documentElement.className.indexOf('wf-active') > -1) {
        document.documentElement.className = '';
    } else {
        document.documentElement.className = 'wf-active';
    }
});

<h1>Demo (green color with a custom font)</h1>
<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer mi ante, vehicula vel mauris ut, maximus commodo quam. Donec nec euismod quam, et cursus metus. Ut ut purus vel neque venenatis pretium. Mauris rutrum mauris nunc. Integer eu elit vel ante mollis venenatis in ut tortor. Duis sodales gravida nibh quis feugiat. Morbi eu fringilla lectus. Duis est dolor, pharetra a ultrices sed, tempor at ex. Nulla gravida eros ac tincidunt gravida. Donec iaculis ipsum eget magna aliquam mollis. Sed sagittis nunc ut suscipit molestie. Vivamus vel nunc facilisis sapien lacinia pharetra id aliquam nisi. Suspendisse vel molestie eros. Interdum et malesuada fames ac ante ipsum primis in faucibus. Sed mollis, eros vel malesuada porttitor, turpis purus dignissim mi, accumsan sodales lectus nisi sit amet justo. Integer hendrerit lacinia diam eget scelerisque.</p>
<button id="switch-button">Switch font</button>

p
{
    font-family: Georgia, Times, Times New Roman, serif;
    font-size: 15px;
    line-height: 1.1;
    word-spacing: 1px;
}

.wf-active h1
{
    color: green;
}

.wf-active p
{
    font-family:'Potano Sans';
    font-size: 16px;
    line-height: 1;
    word-spacing: 2.5px;
}