The main themes could include trust and betrayal. Betty might have initially trusted Dirty John, but he betrayed that trust. Also, the power dynamics in relationships could be a theme. It could show how Dirty John had power over Betty through his lies and manipulation.
In 'Dirty John Betty Based on True Story', some of the main themes are likely to be love gone wrong, the search for truth, and the consequences of one's actions. Betty may have thought she was in love with Dirty John, but as she discovered his true self, she had to search for the truth about who he really was. And the actions of Dirty John would have had consequences not only for Betty but also for those around them.