Did someone say it to you, and what was the situation? In the United States, "break a leg" is an expression used in the theatre, and perhaps in other situations involving stage performances. It is actually a kind wish--an enthusiastic hope for a great performance and lots of applause. "Thank you" is the appropriate response. People have invented stories but I don't think anybody actually knows the origin of the expression. Another stage expression with the same meaning is "Knock 'em dead!"

I do not think it would be a good answer, to say "thanks". If you did you might be going into agreement with the idea to some extent-- and, of course, you do not want that to happen. I think a smile or a "thumbs up" would be a more appropriate response. Of course, you know that the person telling you that does not really mean it-- they are trying to loosen you up by having you face up to the worse that could happen-- but you do not want to agree with that worst case.

I think so! :)

Yes it would be!