The rising action in The Book Thief begins when Death sees Liesel steal her first book after her brother dies. This sets into motion a chain of events that has Death watching over her and taking an interest in the book thief. Liesel stealing books is something that defines her entire life and happens in the most important moments. For example, the first book she steals is one that her foster father used to teach her to read. The rising action as she steals the books from the mayor causes the reader to wonder whether she'll get caught and what will happen. The relationships she builds, the historical events of the rise of the Nazi regime around her, and her time with books all contribute to the rising action. The rising action continues until the climax.
The climax of the novel happens when Liesel's town in bombed. Her fascination with books saves her life, because she's reading in the basement at the time. However, many of the people she cares about die.
One theme of The Book Thief is the importance of relationships between people. Liesel is taught to read by her foster parents. She's given access to more books by the mayor's wife. She's defined and developed by the relationships around her. Without those relationships, she would not have been saved by the bombing, because she would not have been reading in the basement. However, the novel also looks at how death doesn't discriminate. Many people die in concentration camps. Good people die in Liesel's life; so do bad people. Young and old people both die too. The novel itself opens with the death of Liesel's brother. The book is narrated by Death, who makes it clear that no one will be spared; however, knowing that death is coming makes one's actions during life that much more meaningful.
Your question combines two important literary elements: plot and theme. The plot would involve the exposition, the inciting incident, the rising action, the climax, and the resolution. The theme, of course, is the general subject (or general subjects) of the book.
Let us deal with the two elements of plot first: the rising action and climax. The rising action is when the tension heightens in the book. It is usually quite long because it involves everything from the inciting incident to the climax. In the case of The Book Thief, I would say the inciting incident is Liesel’s first attempt to steal a book. This is during her brother’s burial. The rising action begins here and involves everything that follows until the novel's climax. Liesel meets friends like Rudy and Max, as well as the well-meaning mayor’s wife and the gentle Hans. Liesel also steals more books and learns to read them. She joins Hitler’s Youth and thinks deeply about the issues plaguing her country. The tension is really high as everyone in town is herded into bomb shelters. The climax (or highest point of the tension) occurs when Liesel’s town is bombed without warning, causing many of the characters die. Hans, Rosa, Rudy, and other neighbors all perish during this bombing. Liesel is saved only because she was reading in the basement.
In regards to theme, it would be unfair to name only one theme of The Book Thief. There are many general themes that eNotes speaks about: death, love, friendship, war, and family. We could also be very specific and say there are more intricate themes of World War II, the Holocaust, concentration camps, and Nazi Germany. However, if I was forced to name one theme and one theme only, I would say death in Germany during World War II would be a good all-encompassing thematic idea.