How to give different feedback messages for correct and incorrect answers?

Hi Eddy,
there are no separate fields for extra text based on correct or incorrect.

The best you can do is enable the quiz setting “Always show extra content” which will allow you to show that content even when the answer is correct (looks like you already have this enabled).

As a side note – love the design customizations you made with the styler addon. The golden colour fits your theme beautifully.