I'm Gage Kile, and this is what I gathered from "Rumble Fish".
A theme from "Rumble Fish" would be that life isn't fair. Rusty doesn't have a mom, his dad doesn't care about him, and he's left on his own most of the time.
Another theme would be don't judge a book by it's cover. At the end, the Motorcycle Boy dies, and Rusty sort of experiences what he had to live through. Although the MB seemed peculiar, it was because he wasn't always able to express himself clearly, and people thought of him as odd.
Steve told Rusty he'd no longer go around doing stuff with him. This implies that Steve began to look out for himself more, and how he matured through the end.
Rusty stole a bottle of medicine for his pain. That, on top of fighting and drinking, means he's reckless and doesn't really think before he does stuff.
I personally enjoyed Rumble Fish. It was the first high school book to display how tough it is for some people. It had a lot of parts that were intense. I don't think it could be made better, and it's fine as it is.