Change "expand" icon to "contract" icon on expansion

The easiest way is to use CSS for both open and closed accordion arrows. In your themes style.css file you can add something like this:

.accordion-title {
    background-image: url("accordion-closed.png");
    background-repeat: none;
    background-position: left center;
    padding-left: 40px; /* Adjust this based on your images width */
}
.accordion-title.open {
    background-image: url("accordion-open.png");
}

You’ll have to create the two images (accordion-open.png and accordion-closed.png) and add them to your theme as well.

That should do it.