.PostStream-item:nth-child(1)
will target the post rendered at the top of the stream, but due to the infinite scrolling this will change as previous pages are unloaded.
.PostStream-item[data-number="1"]
will target the post with number 1, so that if it's not visible no other post receives the style.
The design of the first post in my Wordpress extension might not be 100% applicable for normal posts because it's not a comment post, it's a special post type which has its own custom HTML template inside.