How does the mood of the story in the Outsiders change throughout Chapter 9?

The mood changes from anticipation to grief. 

At the beginning of the chapter, Pony is anticipating the rumble.  He is not necessarily excited about it, but he is determined to participate.  This is not really a good idea, since he is still not in very good shape from the church fire.  He does not want to seem chicken or weak. 

I swallowed five aspirins, though, when Darry and Soda weren't looking. I do that all the time because I can't sleep very well at night. … I figured five would keep me going through the rumble and maybe get rid of my headache.(Ch. 9) 

Pony does make it to the rumble, where the mood is frenzied and exciting.  Pony is glad to participate, even though he gets hurt and so do most of the others.  

The excitement was catching. Screeching like an Indian, Steve went running across the lawn in flying leaps, stopped suddenly, and flipped backward. We could all do acrobatics because Darry had taken a course at the Y and then spent a whole summer teaching us everything he'd learned on the grounds that it might come in handy in a fight. (Ch. 9) 

Pony is proud of his accomplishment.  He is happy that they win.  Fighting is not really his thing, but he wants to support his friends and he enjoys the hectic excitement of the fight. 

Pony’s happiness is short-lived. He goes to the hospital to tell Johnny that they won the rumble, and learns that Johnny is in worse shape than everyone realizes.  Pony listens to his friend tell him to stay gold and then Johnny dies. 

Dally is horrified and overwhelmed with grief.  He slams into the wall and then runs off.  The joyful occasion of having beaten the Socs in the rumble is gone.  Pony is in a daze after this, and the mood is sorrowful.  That continues into Chapter 10.