On one of my computers, the first post of a thread has a dark background, so it stands out from the rest and makes it easy to make sense of the various threads.

In Tree mode i see the dark background of the first post but not in Flat mode.

All posts look the same on my devices, but once in the thread, whichever post is displayed in the box up top is also highlighted down below.  It keeps track of where you are when moving down and reading all of the posts in the thread.