With "reported speech" the important thing is to explain the information, not the words, to your listener. The information need to be true and make sense to your listener now. In the Original Poster's example, if stealing is against the law now then we can use either of the following sentences:
She was told that it was against the law to steal.
She was told that it is against the law to steal.
This is because the speaker was told that generally stealing is not allowed. This situation was true when the speaker was told, and so sentence (1) is correct. If the speaker was told that it is generally against the law to steal, then sentence (2) is correct as well. The information the speaker was giving was not intended to just be true about that particular time.
Which choice we use though, may be important. If we are trying to show that something is still true now, for example, we will probably use sentence (2). If we are trying to describe a story in the past or a situation in the past we will use sentence (1). Here is an example where it might be important to use sentence (2).
Suppose you want to tell somebody that you have a particular policy about something, and that you have checked this with your boss. Because we are interested in what the situation is now we will expect you to say:
- The Director said that this is our policy.
Now if you say:
- The Director said that this was our policy.
Then this might sound as if your policy has changed now, but it used to be your policy. This implication will be stronger because we expect the sentence to say is our policy, so we might think there is a special reason for you to change the tense here.
Of course both sentences are true and both sentences are grammatical, but in this situation, it might be better to say it is our policy, just because the listener will not be able to misunderstand the sentence.
Hope this is helpful!