Harper Lee uses chapter 27 to transition from what has occured already in the story to how she intends to end it.
This chapter builds suspicion about Bob Ewell. He gets a job and fails, Judge Taylor's house has a suspicious character looming around, and Helen Robinson is regularly attacked by the Ewells when going to work. All of these events work together to propose that Bob Ewell wants to get revenge for the people who worked together to make Ewell look pretty bad at the trial. The only people who had not received any kind of reprecussion from him are the Finches. Then, Scout foreshadows that they are about to have their longest night ever.
I would argue a good theme could be suspicion or building revenge. As far as a main event, I would say that there are 3, but the most important is probably Bob trying to hold a job. If he had found some success in that, he may not have stooped to hurt the other people.