Is a wife's surreptitious taking of money from her stingy husband's wealth, with the intention of performing the Hajj pilgrimage, considered theft and is it forbidden?

The husband is not obligated to provide what his wife uses for Hajj, nor is she permitted to take from his money secretly for that purpose. Rather, it falls under good customs and benevolence.

The husband is obligated to provide for his wife. If he is stingy with her, it is permissible for her to take from his money without his knowledge, to the extent of what suffices her and her children, according to what is customary, as in the hadith of Hind.

Therefore, what you did by taking his money without his knowledge for Hajj is forbidden (haram), because it is taking a Muslim's money without right. You must repent and return the money.

As for whether it is considered theft, if it was a trust or not protected, then it is a breach of trust. If it was not a trust and was protected, then it is theft.

However, if you meant that he used to take from your money without your consent, and you wanted to take your money to perform Hajj with it secretly from him because he would not allow it, then there is no blame upon you. This is known as the issue of al-Dhafr (securing one's right).