One of the themes of Anne Frank's diary is the loneliness and isolation of adolescence. Being a young adult can be hard at times, but it's harder still if you're forced to live in such a tense, claustrophobic environment like Anne. Anne is growing up very quickly, but there is really no one around with whom she can share her experiences. She's not particularly close to Margot; her relationship with her mother is fraught, to say the least; she completely adores her father, but there are certain things that she can't really share with him. So she remains alone. And her feelings of loneliness and isolation are compounded by the fact that no one truly understands her. Anne is very mature for her age, full of complex thoughts and feelings and it's so incredibly frustrating for her not to be able to share them with others. Peter van Daan briefly fills the role of confidant, but ultimately his intellectual inferiority and relative lack of maturity count against him.

There are several themes in the novel "Anne Frank: The Diary of A Young Girl."  One of the themes is the sacrifice of one human being for another.  The act of hiding people at the risk of your own life and giving of your food, living space, and friendship to spare the lives of people around you.

"An underlying theme of Anne's account is man's inhumanity to man. Simply because of her religious beliefs, Anne is confined and lives in constant fear of death. Eventually, she does die, along with over six million other Jews during World War II."

Yet another theme is what happens to people when they are imprisoned.  How do they change?  What happens to people when they try to live, as the Franks did, for two years in a small area with little food, space, entertainment, and no freedom?

The Diary of a Young Girl is a classic coming of age story, maybe not in all the typical senses that we imagine teenagers coming of age, but she certainly goes through all the motions, even in her seclusion.  I agree with the prior answer in spite of the presence of all the oppression and war, Anne demonstrated unfailing optimism and childlike innocence in a situation in which most of us would have succumbed to utter depression and given up hope.  This was not true of Anne who maintained as positive an outlook as possible considering the circumstances.  Some other minor themes might be considered to include the loneliness of adolescence and the generosity and greed of people in wartime.  Check the link below for more detailed information on the themes.  Brenda

I would be sad to think that Anne Frank’s main legacy as a description of a life of oppression.  I think that the main theme is the tenacity of the human soul to continue to grow and even thrive in the worst of circumstances.  Anne Frank is essentially a coming of age story.  She grows from a child to a woman in that attic room.  Yes, the circumstances are unusual and add intensity to the situation.  However, the point is that she grows.  Even locked within an attic room, Anne can grow.  That is a testament to the resiliency of the human spirit.  The Nazi’s could take nearly everything away from her: her possessions, her family, and even her life; but they could not take away her spirit.  She is forced to learn at a young age that happiness and contentment are not based on material goods, or even on relationships, but within our own minds and souls.  She not only survives in the attic room but she creates a diary that speaks to the world in timeless truth. 

I would say that the main theme is living in oppression.  The Nazis have taken away everything from the Frank family (and many like them).  "Jews could not attend schools with non-Jews, go to the movies, or ride on the streetcars." Anne is denied basic human rights and human dignity.  It affects her physically by living in a cramped environment.  It effects her emotionally by repressing her budding feelings for Peter.  And of course, it effects her life by ultimately taking it away. 

There are other themes in the work, but this one affects all areas of young Anne's existence. 

The themes of Anne Frank's diary are developed through the cat-and-mouse relationship between the innocent people who are hiding in the upstairs rooms and the powerful Nazi forces that are searching for Jewish civilians to send to their extermination camps. What gives the diary texture and importance is the deadly game Anne and her family and friends are forced to play. They are in imminent danger of being arrested if they make a bit of noise. They must behave like guilty fugitives, while the real guilty parties, the Nazis, are free to roam the city and intrude everywhere.

What is impressive about the book written by such a young girl is that the fugitives all try to lead conventional lives in spite of the horror of their situation. While terrible events take place outside the rooms, Anne continues to document many of the ordinary parts of her life as she grows up. Anne herself continues to study just as if she were still going to school. She is not oblivious to the danger they are all in, but she insists on remaining civilized in an uncivilized world. The themes of self-discovery and optimism are conveyed as Anne writes about the ordinary struggles and interactions taking place inside her little world. Though they undoubtably face unusual danger, the inhabitants of the annex struggle with normal interpersonal problems such as Anne's sparring with Peter and her conflicted feelings about her mother and sister. Though these minor events may seem unimportant, they demonstrate the unwillingness of the fugitives, including Anne, to let the looming threat of the Nazis completely overshadow or define their own lives. Despite the confining situation in which she must live, Anne manages to grow as into a more thoughtful and perceptive young woman, refusing to lose faith in the ultimate goodness in people's hearts. "The Diary of a Young Girl" has become a classic because of the gravity of the conditions under which it was written. In the end the fugitives are all captured, which shows that the danger was always present and always very real.