What is the theme of The Diary of Anne Frank with three supporting details?

The theme of the story is that you can maintain high spirits even in the worst of situations. 

Anne Frank never lost her faith in humanity.  She was a victim, yes.  She was also a survivor.  Although she eventually died, Anne hid for years from the Nazis trying to live her life on her terms.  It was not easy. 

Anne lived in one of the worst time periods in history, but she still maintained her faith in humanity.  In one of the last entries before the family was captured by the Nazis, Anne writes that she still believes that people are inherently good. 

It's a wonder I haven't abandoned all my ideals, they seem so absurd and impractical. Yet I cling to them because I still believe, in spite of everything, that people are truly good at heart. (SATURDAY, JULY 15,1944) 

Despite how bad things have gotten in the world around her, and the horrible things she has seen people do, this shows that Anne has kept her faith.  This is because Anne had a generous spirit.  She was a boisterous girl who wanted to be a writer.  She shared her intimate thoughts in her diary, and many of them had to do with feeling misunderstood.  Despite all of this, Anne still wanted to see the best in others.