You’re right that today the term “flattering” is mostly in the context of slimming or “to look thinner,” but it used to mean to enhance your natural beauty. For instance, when you use make up to enhance you’re naturally pretty eyes you use color or shading, you enhance lovely lips by daring attention to them. You can do the same thing by enhancing some really great curves with a belt, it’s “flattering”.
When I think of personal style I think of the woman I want to be -what would she look like?- and my fashion role models. I would love to dress in a combination of Elementary’s Irene Adler/Moriarty and any role Angelina Jolie plays. Strong women. It’s not just about the clothes you wear, because personal style also includes attitude. That’s something to keep in mind.