How to set class on last thread reply?

Hi,
I have a requirement, when you click a thread page, there you can see many reply on thread/post. But I want to see first thread design should look different then other.

Solution: I have set the first thread design using thread parent class and it works.

Complication: When user sorted that "oldest to newest" all design get shattered because now post went to last position and this class will not consider parent class.

What I exactly want.

I want to set the class on last thread but it should not impact the first thread as well because same class is using in first and last thread. On this thread there is sort option available. When I sort it to "oldest to Newest" It will come on position one in that case it looks fine but when I sort it to "newest to oldest" then it goes to last position and design get shattered, because now its not parent class anymore.Also there is pagination on thread sometime it pages may be 2 some time more than 50. So I think its not feasible with js as well because we can't get the last page because of pagination. There may be many pages.

Any idea how can I set the extra element on same class in different pages?

Regards,