Hi Sarah,
I would think judging by the tempretures they are having over there, thermals would be favourable, ive been told its better to layer up, so you can take off, than to not have enough on and freeze.
Im going to wear thermal vest, polo top, fleece and ski jacket, and see how i go
was in Borovets last week, temp droped to minus 20,
you will need thermal base layers, fleece jumpers, hats and gloves whilst skiing.
Best not to wear cotton tshirts as base layers as they can get damp with persperation and unlike proper base layers tshirts wont dry out and if you are wearing them against your skin you will get very cold.
Go to somewhere like Millets and look for Thaw or peter storm ski base layers, quite cheap and very good for keeping you warm and dry.
Kel