Another theme popular in Rowling's work would have to be the necessity to break rules. In every Harry Potter book Harry and his friends break the rules for the greater good. In the first book they go to the third floor corridor and to the Forbidden Forest which are both forbidden. In the other books there is also the theme of rebellion against government. In Harry Potter and Half-Blood Prince Harry defies the newly appointed Minister of Magic and speaks rudely to him.
A well known theme found in the books would have to be the importance of loyalty. In Harry Potter and the Prisoner of Azkaban we find out that Sirius Black did not betray the Potters but that Peter Pettigrew did. Harry feels personal hate for Peter because he betrayed his family when they trusted him. Loyalty and trust is the core of all the Harry Potter books.
Another popular theme would have to be the injustice of the legal system. In Harry Potter and the Prisoner of Azkaban we learn that Sirius was sentenced to a lifetime in Azkaban without even having a trial. It turned out Sirius was innocent and was unfairly put to suffer with the Dementors. Another example of injustice would have to be the sentencing of Buckbeak the Hippogriff to death. When Hagrid went to defend Buckbeak he said that Lucius Malfoy was on the opposing side. It was concluded that Malfoy ahd some input in the decision and tsince he has money the coucil sided in his favor.
Probably the most important theme would have to be tolerance of others. In the wizarding world many prejudices exist: purebloods against muggle-borns, humans against beasts, and rich against poor. Probably the most prodominant would have to be anything concerning blood. In the series there is a war going on between those who see themselves superior