It's very hard to come up with a theme with a book like this, but I think that people like Deborah, with mental problems, need people around them (like Carla, her parents and Dr. Fried) to take care of them. So that would make the theme sort of 'friendship' and most importantly trust (Deborah trusts Dr. Fried).
Another theme could be; learn to handle the ups and downs in live and that live isn't always that good.
It is set mostly in a 'madhouse' in Victoria (America), around 1948, because she was born in 1932 and was sixteen when she got in the 'hospital'. She got out, at least the book ended, at the age of 19.
Deborah Blau: A very bright girl with a gift for drawing, who's mentally very ill, because of her past. That's why she lives in two different worlds; the real one and a world called Yr.
Dr. Carla Fried: A well-known doctor who treats Debby and cares about her. She understands Deborah well and helps her out of the Yr-world.
Esther and Jacob Blau: parents of Deborah. Esther is certain about putting Debby into the hospital, but Jacob isn't so sure and doesn't want to admit Deborah's illness.
Carla: Debby's friend in the hospital. They understand each other well.
Suzy: Debby's five years younger sister. Sometimes a bit jealous of the attention Deborah gets.
The doctor of Deborah, Dr. Fried, tells her that she never promised her a rosegarden. Which means that live isn't only sugar and candy, but that there are bad and ugly things too. And Deborah will have to face that if she will live in the real world.