If you were fired for theft and interviewer asks why you were fired how do you answer that?

In a case like this, you should probably omit the truth without lying (if that makes sense). In other words, you should tell the interviewer that you were let go for dishonesty but are working towards it now and then explain what you are doing to work towards this. It is your right not to give details about the situation but be aware that they may check with the previous employer.

And if I find that you did not honestly tell me about being fired for THEFT, not mere vague dishonesty, I will never hire you.