Since I'm not very well - acquainted with this specific 'lark to lust fan fiction', but generally in fan fiction, common themes could include transformation, discovery, and relationship development.
I'm not sure about the exact common themes in 'lark to lust fan fiction'. However, if we consider the two elements 'lark' which might symbolize something pure or innocent and 'lust' which is about strong desires, themes could revolve around the contrast between purity and passion. Maybe there are themes about the breaking of taboos, the exploration of new feelings that go against one's initial nature, or the struggle between one's innocent self and the allure of more intense emotions. Fan fiction often plays with such contrasts and developments in characters and their relationships.
As I don't have a lot of knowledge about 'lark to lust fan fiction', but thinking about similar types of fan fictions, themes like self - exploration, the power of emotions, and the journey from innocence to a more complex state might be present. For example, in some fictions, a character starts as naive (like a lark) and then goes through experiences that lead to a more passionate or lust - filled state, which could involve themes of growing up, facing new feelings, and dealing with social or personal boundaries.