My work doesn’t have a dress code. So far as the organization is concerned, professionalism isn’t actually hampered by the business casual dress attire, but in regards to client interaction, true professionalism would indicate following the expert dress code. Maybe you never needed to think of specialist dress outside your jeans for school, work clothes for a lab, or a different job that didn’t need expert attire.
Don’t make the error of wearing baggy and shapeless clothes if you’re a little on the heavier side. Casual jackets are usually worn outside to provide a small additional warmth to a casual outfit. Lots of people would never think about a silk scarf in the winter, but silk has ever been an incredibly warm garment.