what is capotes theory and how does he convey his theory?

This question definitely doesn't warrant a short answer/ You might want to begon by reading through gradesaver's study guide for this novel. The direct link is provided below; this section deals directly with Capote's experiences in writing this novel. I would also go through the summary.

http://www.gradesaver.com/in-cold-blood/study-guide/about/

If you're still having problems, I have also linked another site below that you may find helpful;

http://www.sparknotes.com/lit/incoldblood/section10.rhtml