As in all of the Harry Potter books, the overwhelming theme is good vs. evil. Voldemort and the Death Eaters represent the "evil" aspect of humanity, while Harry and his friends represent the "good". However, in this book, Rowling challenges this theme to make it more complex. Through Dumbledore, she demonstrates that people can be very "good" and make bad friends and bad decisions that they later regret. Dumbledore is swayed by Grindevald's theories of wizard dominance, and suggests grabbing power for "the greater good". He later learns that the ends do not justify any evil means. Harry is "good" throughout, but he still learns this lesson through his struggle to accept this new information about Dumbledore.
Another strong theme is selflessness. Harry makes the ultimate sacrifice in this book, walking forward to die in order to save others. He accepts the choice willingly. Neville fits against the evil regime at Hogwarts because he knows it is better for everyone if Voldemort is challenged, even though he could be easily left alone (he is a full wizard, and therefore not a target). Hermoine gives up her parents, knowing that they will be safer far away, without any memory of her.
A theme for "Harry potter and the deathly hallows" is "Sacrifices are needed for victory" because lives were forfeited in the book in order to defeat Voldemort. Another is "Perseverance" because even when times were tough and people died, the characters moved on to continue fighting.
One of the major themes in Harry Potter Book 7 is loyalty. Help will always be found in Hogwarts for those who seek it. The castle takes in all the rebellious students and provides them a safe house from the Death Eaters via the Room of Requirements. Neville promises Harry that he will try to kill Voldemort's snake--and he does. Ron leaves Harry and Hermione but comes back when they need him the most. Snape stayed true to the very end and protected Hogwarts and carried out Dumbledore's will. These are just but a few of many examples of loyalty in the Deathly Hallows.
The Themes are:
1. Good over Evil - Harry defeats Voldemort
2. Friendship - especially between Harry and Ron when Ron gets angry
3.Love - Harry and Ginny, Ron and hermione, remus and tonks, etc.
4. Loyalty - harry towards dumbledore, dobby and kreacher towards harry
5. Death - Fred, mad-eye, lupin, tonks, pettigrew, etc.
6. War - Acc. to JKR in an interview, war is a predominant theme. One moment you're talking to your friend, next - he's dead - like fred.
7. Prejudice - against house-elves and muggle-borns
the themes in harry potter and the deathly hallows is the horcuxes that they have to destroy on the journey.
Among many others, JK explores the following in DH:
LOVE-Harry's love for Ginny, Ron's love for Hermione, and even Lupin's love for Tonks all play a vital role in the novel
Sacrifice-both Dobby and Harry explore what it truly means to sacrifice oneself for those they love
Friendship-probably the biggest theme, this one runs throughout all of JKR's novels. From Ron leaving Harry and Hermione (only to return to them when they needed him most) to Luna's bedroom decorations, this theme underlies all others.
Good versus evil-the really obvious one: Voldemort versus Harry (and everyone else, too, of course)
**Not making assumptions-your basic "what makes someone good or bad" stuff here