Is taking money from one's parents without their knowledge, by slipping away leftover cash from their purchases, considered theft, rendering the money unlawful, especially since this taking occurs without an announced allowance? And is there even such a thing as theft from one's parents in the first place?

Theft from parents is a greater sin than theft from others, because it combines disobedience to parents (ʿuqūq) with theft. Taking the remaining money without the knowledge and permission of the parents is considered a betrayal of trust and an unlawful consumption of wealth. Trusts must be returned to their rightful owners. This act must cease. As for what was previously taken, permission must be sought from the parents. If they grant permission, then all is well. Otherwise, it must be returned to them. If the parents are spending on your necessities, they have fulfilled their obligation. Daily spending on non-essentials is not obligatory, and the obligation of your parents' spending on you arises when you are unable to earn and do not possess money.