What is the most significant theme of "That Was Then, This Is Now"?
That Was Then, This Is Now contains many themes that could be considered the "most significant" theme. Individual readers are going to see certain themes as more central than others. That makes the answer to this question a matter of opinion, so feel free to pick a theme and adequately support it.
For me, the theme that stands out as most significant is the coming of age theme that is present within this story. There are themes about friendship and family within the book; however, I feel that Bryon's coming of age is what affects those relationships. That's why I feel the coming-of-age theme is more significant than the relationships theme.
As Bryon matures throughout the story, he realizes that he has to "grow up" and get himself on a more straight, narrow, and legal path. That's why he gets the job at the supermarket. He has realized that he must help support his family financially, and he no longer considers pool hustling a viable way to earn a living. His desire to do things legally is also a part of why he is so appalled when he discovers that Mark has been selling drugs. The end result is that Bryon turns in his best friend and foster brother to the police. Mark is sentenced to serve jail time, and the relationship is broken forever. Bryon has learned that growing up and doing the right thing is not always easy and won't necessarily have a happy ending.
An additional force that drives a wedge between Bryon and Mark is Bryon's relationship with Cathy. As Bryon falls more and more in love with Cathy, he grows more and more distant from Mark. Bryon realizes this is happening. He's saddened by the distance growing between Mark and him; however, Bryon understands that his relationship with Cathy is part of growing up and is also good for him.
I had quit thinking only about myself.
Unfortunately, Mark can't handle the growing divide, and he harbors some resentment and anger toward both Cathy and Bryon.
S.E. Hinton examines several themes throughout the novel That Was Then, This Is Now, but the most significant theme deals with relationships. She explores relationships—particularly Mark and Bryon's friendship, and Bryon and Cathy's romance. Hinton depicts how close friendships can dissipate over time, and romantic relationships can change. For the majority of the novel, Bryon and Mark are best friends who do everything together. As the novel progresses, Bryon falls in love with Cathy and begins to spend less time with Mark. Bryon also begins to realize Mark has no conscience and does not have an understanding of right and wrong. When Bryon finds out Mark has been selling pills, he calls the police on him. By the end of the novel, Mark and Bryon are no longer friends, and Mark says he hates Bryon. After Bryon calls the police on Mark, he is emotionally drained and breaks up with Cathy. Bryon once loved Cathy, but after his friendship with Mark ends, Bryon is no longer interested in her. Throughout the novel, Hinton depicts how traumatic events and difficult circumstances negatively affect relationships.