Student or Learner
The only dress I’d brought was a white sundress with blue flowers on it. It was pretty, but I thought something a little more sophisticated might be called for. Using my magic, I changed it into a simple black, sleeveless sheath. I had added a little black shrug jacket and pearls for good measure before it occurred to me that I was using my powers again.
"and a pearl necklace to finish the outfit off"
"for good measure" - as an extra precaution, or beyond requirements, in addition.
In this instance she's added the jacket and necklace to make sure the outfit is sophisticated enough, above and beyond the classic black dress.