I am no fan of javascript. In fact, I despise it. But, recently, it occured to me that it would be cool to change the CSS theme according to the client's time.

Here's how:

function change_css_according_to_time() {
    var d = new Date();
    var h = d.getHours();

    if ( h >= 7 && h <= 19 ) {
        console.log( 'Good day!' );
        document.getElementById("mainStyle").href = '/css/default-day.css';
    } else {
        console.log( 'Good evening!' );
    }
}

change_css_according_to_time()

Check out the default.js. It's as simple as that.

Also, you just need the VanillaJS framework: http://vanilla-js.com/.

Have fun!