One possible theme could be betrayal. If the mother has a lover, it likely involves a betrayal of the family unit or her relationship with the father figure. This can lead to a breakdown of trust within the family.
Love, of course. It's in the title, so it's likely a central theme. Maybe also the idea of a journey, as it's a 'path'.
The themes might include loyalty. The CEO may have to deal with issues of loyalty in his business world while also maintaining loyalty in his relationship. Redemption could also be a theme, perhaps the CEO has made some mistakes in his past, and through his relationship with his lover, he seeks redemption. And of course, the theme of sacrifice, as either the CEO or the lover may have to sacrifice something for the sake of their relationship.
If it's a positive fanfiction, possible themes could be the mother - daughter relationship, like how the mother passes on her wisdom to Sakura.
One possible theme could be secrecy itself. The fact that it's a secret lover implies that there are things that need to be hidden, like perhaps the relationship goes against social norms or workplace ethics.
One possible theme could be self - discovery. The mom might have discovered new aspects of herself while falling in love during vacation.
I can't really say for sure as I'm not familiar with this specific fanfiction. But generally in fanfiction, themes like love, family, and self - discovery can be common.
I'm not sure which specific 'her mother's lover novel' you are referring to. However, some classic novels with complex family and relationship themes like 'Wuthering Heights' might be somewhat similar in exploring intense emotions and complex relationships within a family context.
Another theme could be about redemption. Maybe the character Landon is trying to redeem his or her lover who has a dark past as a killer. It could be a story of trying to change someone for the better.
Another theme could be acceptance. The human in the story has to accept the fact that their lover is a ghost, with all the strangeness and limitations that come with it. Also, the ghost may have to accept that the human has a different way of life. There might also be a theme of loneliness. Maybe both the human and the ghost were lonely before they found each other, and their relationship fills that void in their existences.
One possible theme is secrecy. Since the lover is in the attic, it implies that their relationship has to be hidden. Another theme could be love against all odds. They are willing to face the difficulties of the situation just to be together.