The villainess herself is often a popular character as readers are interested in her journey from being 'bad' to potentially 'good'. The male lead, who is usually involved in the villainess's romantic arc, is also popular.
One common theme is the transformation of the villainess character. Usually, she starts as a disliked or misunderstood figure but gradually changes for the better. Another theme could be romance. Often, there is a love interest involved that helps in the character's development.
I'm not entirely sure as I haven't read it in full. But generally, it might be about a villainess who tames a beast. This could involve a story of power dynamics, where the villainess has to use her wits and charm to subdue the beast, perhaps in a fantasy world filled with magic and adventure.
Since I haven't read it, I can't say for sure. But it's likely that the villainess herself is a main character. And of course, the beast she tames would also be a key character.
Another theme might be subverting traditional storytelling. By favoring the villainess, it goes against the normal narrative where the heroines are always the 'good' ones. This can lead to exploring different moral and ethical concepts.
The unique plot. It flips the traditional story where the protagonist usually falls for the hero. Here, falling for the villainess is a fresh concept that attracts readers.
The commoner she wants to marry is also crucial. He represents the 'other' in the story, from a different social stratum than the villainess. Their interactions and relationship development are central to the story.
Another theme might be self - discovery. As she tries to marry a commoner, she may learn more about herself, her desires, and what truly makes her happy outside of the expectations placed on her as a villainess.
The protagonist who favors the villainess can also be quite interesting. Their reasons for siding with the villainess, whether it's because they see something special in her or they want to change the story's course, can add depth to the character.