When You Reach Me by Rebecca Stead: Theme of the book

*WARNING: THIS ENTIRE POST IS PRETTY MUCH A SPOILER!!!!!*                                                             While reading this book, I thought a whole lot about what the theme is. I have slight idea, but it's a bit confusing. The story is about a girl named Miranda, and it takes place in the 70s. One day, when they're walking home from school, her best friend, Sal, gets randomly punched in the face by this kid, Marcus, who hangs out with his friends in a parking garage. And also there's this bum who lives on a bench on the corner of the street. Anyway, so after that, Sal doesn't want to hang out with Miranda anymore, for what seems like no reason. So Miranda finds two new friends, Colin and Anne Marie. They get a job at a cute little diner, and they work there at lunchtime. But Anne Marie has her ex-best-friend Julia following her like a puppy, and Miranda really doesn't like Julia, she thinks she is pretentious. So when the owner of the store finds out that some of his stuff is missing, he immediately thinks it was Julia who stole it, not because she's pretentious, but because she's black. This completely outrages the three kids, and they never even step foot into the diner again. Then, Miranda starts to actually become friends with the kid who punched Sal, Marcus, and, as to not give too much away, there is a sort of time-traveling theme involved and it turns out that the bum who lives on the bench is actually Marcus from the future. He came from the future to save Sal from getting hit by a truck.
               So because of those two events, I think that standing up for what's right, and defending yourself and others is a big theme in this book. Even though Miranda didn't like Julia, she still stood up against the racist man at the diner. The future Marcus wanted to make up for punching Sal by saving him from the truck. I think that the fact that this book takes place in the 70s takes a major part is this theme of the book. The Vietnam War took place in the 70s, and there was a lot of people protesting against it, wanting peace, and that's exactly what is displayed in the book. There is no clear mentions of the actual war taking place in the book, but once or twice it mentions the year it takes place in. When I first noticed the year, it really sort of made the book more understandable to me. At first it din't really make sense at all.
                So hopefully you'll understand what I'm trying to say even through my poor wording and horribly confusing first paragraph.
                                                                  ~~~~~Lulu