How must a woman dress in front of her husband during her days of niddah?

I have heard that during niddah a woman must follow the deoritah rules of tzinius but not necessarily the rabbinic ones. If this is the case then why, for example, is a tshirt fine when it goes above the elbows?

Is there a general principle with regards to how a woman should dress during niddah in the prescence of her husband?