I don’t know if this is too late since your interview is tomorrow. I work in an office everyday so I know how to look professional.
I would dress in all black, which always looks very professional, just black tailored suit pants and a long sleeve black top. You could wear a suit jacket but it is very un common for officers to be so professional. Then wear a really bright Scarf with a bright handbag.
The key to looking professional and looking a little more grown up is wearing lipstick and your hair tied up!
Let us know how you go.