To accomplish this, you will need to know which is the last post on the page. There are a couple ways to achieve this, but I think the easiest way is to use pure php, the function end(). You are going to use this to get the last post in the loop.
Once you have the last post in the loop, you can simply just compare the last post’s ID with the current post ID, and if they match, you can apply your custom styling
In your content-*.php, you can try the following
$last_post = end($wp_query->posts);
if ($last_post->ID == $post->ID) {
//Apply your styling for the last post
}