She is sitting in the library, studying. It's her most common pastime, by hours; spending time with her sister is more enjoyable but Jaromira prefers to socialize more broadly and Katarzyna would only get in the way. This is better on net; the two of them share a room, so it isn't as though her sister's time is entirely hogged by the other girls who call this boarding school "home." Or at least "prison." Katarzyna doesn't mind it, though; the school library is excellent, and basic manners and care for the books has endeared her to the librarian to a sufficient degree to lubricate the interlibrary loans process when there's something she wants to read that they don't have.
This looks like something she wouldn't mind people seeing, at least. She's not going to get anything really impressive off her own sense of style; anything really interesting is going to need Jaromira's help.
But she can thread silver in with the blue embroidery, and turn her hair silver, and make her feathers silver with a gyrfalcon pattern of crescent stripes on the primaries in opal blue.
...Huh. Well. Perhaps it does something else under other circumstances.
Still.
She just did magic!!!
Time to go find Jaromira! Her sister has an actual sense of style, this is just the tip of the iceberg.
She leaves the bathroom and then the library altogether. She will probably get reactions from people in the hallways who are not her sister, but she can just ignore those.
"I know!!! Come on, let's go somewhere a little more private, I need you to advise me, insofar as I have aesthetic sensibilities the magic seems most unimpressed with them, I suspect you'll do much better."
"Sure," she agrees, giving the girls she was with a brief wave before linking arms with her sister and striding off.
She had been hanging out in the dorm area anyway, so it doesn't take long to get back to their room.
She inspects her sister's transformations so far.
"Try a circlet," she suggests, "in titanium anodized the same blue, with a seafoam opal centerpiece."
"Good. Now modify the circlet so the metal is in a filigree pattern that matches the embroidery on the dress."
"Good. Now...hm...you'll need to do something with that hair. Can you give yourself a thin crown braid for the circlet to rest on? And--hm. Make it look like your hair is gently wafting in some breeze?"
"Okay, I know I've seen magical girls with crazy hair...can you make your hair unaffected by gravity?"