I would say that one theme is authoritarianism. That's not a tough theme to explain. Trujillo is in control of a complete dictatorship and rules with an iron fist. That kind of government permeates the daily lives of all of the characters in the book. But it especially affects the Mirabal sisters, because they are working against that regime. They must carefully watch everything that they say and do or risk their lives.
I think a second theme is the theme of women in political leadership roles. There are times in the novel when the Mirabal sisters struggle with their dual role of being traditional women that stay out of political issues and helping to lead the rebellion.
A third theme is theme of courage and heroism. The Mirabel sisters are incredibly brave women. They are throwing off cultural norms by not staying at home and keeping out of public, political events. Their actions at times are quite public and put their lives at risk. Trujillo knows about them and is constantly looking for a way to eliminate their influence. What I like about the heroism that the Mirabal sisters demonstrate is that their heroism is frequently intertwined with normal mundane life. If this was a Marvel movie, the hero would just be blowing up a bunch of stuff . . . the entire time. That's not so in this book. The sisters' heroism is shown in conjunction with their daily lives of marriage, dating, and being a parent.