Does a son taking money from his mother—money that she took from his father without his knowledge for their marriage—constitute theft, given the mother's fear of informing her husband due to her fear of divorce and his demanding the money back?

You are not permitted to take money from your mother and benefit from it without your father's permission, especially after you became aware that she took it without his knowledge. The money must be returned to the father. This can be done by devising a stratagem without informing him of the reason for the return or who returned it, because it is obligatory to restore a right to its owner. You should advise your mother and help her return the money, as this is part of showing her dutifulness. Your mother must repent, seek forgiveness, and return the money to her husband by any possible means, based on the Prophet's (peace and blessings be upon him) sayings: "The wealth of a person is not lawful [to take] except with the willing consent of its owner," and "On the hand that took it rests the responsibility until it is returned," and "None of you should take his brother's belongings, neither in earnest nor in jest. And if any of you takes his brother's stick, he must return it to him."