Is it permissible to take small amounts from the shop's daily revenue without the partner's knowledge, especially since I work there without pay and am in need of expenses, or is that considered theft and a breach of trust?

There must be a clear agreement on the wage before commencing work; otherwise, the worker is not entitled to a wage unless it was stipulated or became customary. If no wage was agreed upon and no custom was established, then taking money without the owner's knowledge is considered theft, and one must repent and return the money. If the questioner customarily works for a wage for people like the owners of this shop, then he is entitled to a customary wage. If not, then he is not entitled to anything because he is volunteering his work. If the questioner's right is established and the shop owners do not accept it, he may take it without their knowledge (recovering one's right). Otherwise, the amounts must be returned. One must be wary of minor sins.