Sorry but how is it impractical? OP only has to waft around enjoying herself, it's not like she will be running any races.
I would say cream and/or light green for headwear plus your warming layer. Try pashmina, shrug, cardigan style or unstructured jacket.
Re headwear you really just need to find something that suits you, because not all heads (and faces) suit all hats and fascinators, by any means!
Beautiful dress OP. Enjoy your swanky day🙂