what is the main theme of duchess of malfi?

Disguise—masking reality, hiding one’s true intentions, presenting a false front—is a major theme in The Duchess of Malfi. The most obvious symbol of this is Bosola. The distinction between what he says and how he acts is so vast that even the audience, who is given access to his private thoughts through soliloquies and asides, has trouble understanding his motivations. There are a number of other themes that you can explore at the link below:

https://www.gradesaver.com/the-duchess-of-malfi/study-guide/themes