Theft by bailee is a person who holds another person's money and steals it. If you are an authorized user on another persons account but it is not your money and you use it for yourself, it would be theft by bailee. However, as a bailee you are usually authorized to use the money. It really depends on what you use it for.
Without more facts, its difficult to determine whether you would fall under the statutory definition of 'bailee' as that term is used in Wis. Stat. sec. 943.20, which criminalizes thefts. Having said that, if the amount is under $2,500 the crime would be charged as a misdemeanor, and over that amount the crime becomes a felony; again the class of felony and the penalty depends on the amount. And if the money or property is taken from "an individual at risk" the crime can be charged as a felony regardless of the amount. In any event, if you have become a 'target' or a 'suspect' regarding a theft you should immediately consult with an attorney - and if you in in the position of having to listen to Miranda Warnings you should exercise your right to remain silent and to have an attorney present.