Styling specific accordion item

There are a few ways you can do this.

You can add id="last-accordion" to the last [accordion-item] shortcode. Then you can use #last-accordion in your CSS however you see fit.

An easier way, without having to edit the accordions themselves, would be to use well placed borders and padding. For example:

.accordion {
	/* Add a border to the bottom of the main accordion box */
	border-bottom: 1px solid #aaa;
}
.accordion .accordion-title {
	/* Add borders to all sides (except bottom) of the title boxes */
	/* Also add some padding around the titles */
	border: 1px solid #aaa;
	border-bottom: none;
	padding: 10px 15px;
}
.accordion .accordion-content {
	/* Add borders to the left and right sides of the content boxes */
	/* Also add some padding around the content to match the titles */
	border-left: 1px solid #aaa;
	border-right: 1px solid #aaa;
	padding: 0 15px 10px;
}