how can i change mybb thread icons?

Okay i googled around to find a way to change thread icons. But all it shows is how to change ficons etc but not how to change the icons off posts. I wanted to swap them with font awesome icons but no clue how to do this and i cannot find a tut myself.

some tuts i found which do not show how to change thread icons.

https://community.mybb.com/thread-156711.html