Your question isn't really fair because of the large number of themes in this coming-of-age story: basketball, drugs, friendship, family, education, attitude, etc. However, if I had to choose a MAIN theme, I would say, by FAR, the main theme is as follows: TEAMWORK.
If you look at all the minor themes of this bildungsroman, you will see teamwork is involved in ALL of them. One cannot play a team sport like basketball without teamwork. One cannot be involved in a business (even an illegitimate business like drug dealing) without teamwork. One cannot be in a healthy friendship or a healthy family without teamwork. (I had to laugh at that last one because I did have to insert the word "healthy.") One cannot receive an education from teachers/mentors/professors without teamwork. And, finally, you cannot achieve ANY teamwork without the proper attitude of respect for all.
Of course, all of this teamwork has to do with the main character of Greg: a fantastic, young, teen basketball star who wants to eventually make it into the NBA. The conflict of talent vs. reality is real here. Hard work is paramount especially within the setting of urban experience. All of this involves teamwork on the part of Greg, his friends, and his family. Greg has to spend a lot of time balancing his "want" of attitude with his "need" for teamwork and, in that, success. You can see this struggle in the quote below:
I'm six four, and I got the moves, the eye and the heart. You can take my game to the bank and wait around for the interest.
In the end, of course, Greg learns that "moves" and height and even talent are not enough for the success and happiness that he years for. THAT kind of happiness only results from teamwork in an atmosphere of respect.