GOOD QUESTION... So far I feel like the costumes to present her lore the most are her high tiers and Lorelei? It's hard for me to pick exactly which one so I will delve into all I know yaay:
- Basilissa is a great costume that showcases her theme of rebirth, similarly like in canon, both dissapear (canon is Sangria parting ways with Nicholas), yet both return to their rightful stages, whether as a queen on her throne or performer in an opera house. They both are also supported by women in red (funnily enough Keigan's middle name is Nicholas too!), and it feels like an essence that reverses the roles and gives her the strength and power. Before she was a tool to her father, indoctrinated by Nicholas, but she is finally given a control she typically never had. . .
- Voice Beyond the Stage (I will call her vbts for simplicity hjfhbds) is a fun costume that's more direct about her role as an Opera performer? But once again she is given strength and power she usually didn't have. I have talked about this but I feel she is a mixture of Sangria and Nicholas, she is a performer trapped and used, but also somebody whose words become important to the fate of the story, and what I really like about VBTS's color pallete is she is bright red, a great contrast as she is canonically described as a dull grey...she really feels like the Nicholas to Vera in this story... just. less cultism.
- I need to reread Midnight Carnival BADLY but I like how it acknowledges the """accident""" in her canon lore that resulted in multiple deaths in the opera house. Yet, she may or may not be treated as innocent this case scenario. At the end she also said she wouldn't perform until she helps a friend.... I need to reread to know for sure if it could be about Alva or somebody paralleling Nicholas....

TL;DR: I feel her current skins all reflect a certain element, Nicholas, the opera house's "accident", or her Rebirth... But I need to theorise more on Pawn Sangria. Pawngria if you will.