I am not sure if this is the responsibility of Commerce or Commerce Coupon, but I'll start here.
It is currently not possible to theme the discounted amount in the cart views (for example Commerce Cart Summary) because of the fact that the surrounding tr has the class:
component-type-commerce-coupon-pct-"actual code of applied coupon"
Since the coupon is most likely to be unique, there's currently no way to theme the discounted row on the cart summary. For example, it could be useful to make the specific line in a red color.