Is it possible to listen to a "style change" event?

The declaration of your event object has to be inside your new css function. Otherwise the event can only be fired once.

(function() {
    orig = $.fn.css;
    $.fn.css = function() {
        var ev = new $.Event('style');
        orig.apply(this, arguments);
        $(this).trigger(ev);
    }
})();

Since jQuery is open-source, I would guess that you could tweak the css function to call a function of your choice every time it is invoked (passing the jQuery object). Of course, you'll want to scour the jQuery code to make sure there is nothing else it uses internally to set CSS properties. Ideally, you'd want to write a separate plugin for jQuery so that it does not interfere with the jQuery library itself, but you'll have to decide whether or not that is feasible for your project.

Things have moved on a bit since the question was asked - it is now possible to use a MutationObserver to detect changes in the 'style' attribute of an element, no jQuery required:

var observer = new MutationObserver(function(mutations) {
    mutations.forEach(function(mutationRecord) {
        console.log('style changed!');
    });    
});

var target = document.getElementById('myId');
observer.observe(target, { attributes : true, attributeFilter : ['style'] });

The argument that gets passed to the callback function is a MutationRecord object that lets you get hold of the old and new style values.

Support is good in modern browsers including IE 11+.