Feeling Sorry for Celia - theme

In the book Feeling Sorry for Celia, I think there are a couple themes. One of them could be that “it doesn’t matter what other people think of you, it just matters what you think of yourself”. At the beginning of the book, the main character Elizabeth keeps getting notes from all these clubs like “The Association of Teenagers” and “The Society of People Who Are Definitely Going to Fail High School” that tell her that she is an embarrassment to teenagerhood and that she is going to fail because she is not doing any homework or studying. She is always feeling really self conscious, and never really talks to anyone except her best friend Celia. But at the end of the book, when the clubs are saying mean things about her in the letters, she finally writes them back and says that her life is going great, and the next time they write to her she will flush the letter down the toilet instead of reading it.
Another theme this book could have is “When one door closes, two others open”.
Elizabeth has some hard times in her life, but always seems to find a bright side. Like when Celia ran away to join the circus and was gone for a really long time, Elizabeth would write to her pen pal Christina, and later on they became best friends. When Elizabeth starts to really like this boy named Saxton and then he asks Celia out, she starts getting these anonymous notes from a guy from Christina’s school who has a big crush on her. Every time something bad comes into Elizabeth’s life, she always finds something positive.
A third theme could be “You don’t need a lot of friends to make you happy; you just need one really special friend.” At the beginning of the book, Elizabeth only has one friend, which is Celia, and they do everything together. But when Celia runs away, Elizabeth is left by herself with no one to talk to. She tries to make friends with other girls in her school, but she doesn’t find any that she would really like to hang out with. So she just keeps writing to her pen pal Christina about how she misses Celia and is worried about her. And that makes her feel better. The two are always writing to each other, and they become best friends.