Does the partner alone bear responsibility for stealing money from the car after leaving the mosque, or do all partners bear this responsibility?

The company's money in the partner's hand is a trust (amanah), so he is not liable for it except in cases of transgression or negligence. If the money perishes without transgression or negligence, such as being stolen, the loss is borne by all partners. However, if they did not mix the money, then its loss is borne by the owner of the money.