What are the main themes in act 1 scene 1?

Corruption definitely works as a theme.

Below you will find the first paragraph of gradesaver's summey for Act 1, Scene 1. Both the chapter summaries and the section on theme should help you with your questions.

"At the Duchess’s palace in Malfi, Delio welcomes Antonio home from his trip to France, and asks him how he liked it there. Antonio says, “I admire it,” (1.1.5), for the prince had rooted out all the sycophants and corrupt people in his court to prevent such corruption from spreading through the land, and so was a good role model. Antonio tells Delio that Bosola, who has just entered with the Cardinal, rails against vices, but only because he can’t afford to commit them himself."