There could be a couple themes to the book, but I think the main theme is the conflict between the wilderness/wild and civilization/society. The entire book is about the development of how the dog adapts to his environment in order to survive. If you read Jack London's other works, which I have only read one or two, he focuses on nature a lot. Oh, and by the way, to add on to Isler's grammar correction... the reason why it is "written" instead of "writted" is because "writted" is not word.

As you said.^^ It's a story which happened during 19th-A.C. gold rush. Buck(the dog) was stolen and taken to Alaska,and became a sled dog. He met some bad humans and had a difficult time,almost be killed by fatigue and hunger.But later a kind man saved him and let him know what's called "love". Unfortunately,the man died,killed by some Yeehats（native Americans）.Buck felt very sad.He killed the Yeehats and joined the wolves. In the end of the story,he became the chief of the wolves. Jack London wrote another novel about a wolf becoming a dog. Also awesome.It's called "White Fang".(But it has a happy ending.) By the way,your sentence looks weird. It should be: What's the theme of the book "The Call of The Wild" written by Jack London?