Wis. Stat. § 78.15
If any licensee sells any motor vehicle fuel upon which the tax imposed by this subchapter has become due and has not been paid, all motor vehicle fuel tax moneys received by that licensee because of that sale are trust funds in that licensee's hands and the property of this state. Any licensee who fails or refuses to pay over to the state the tax on motor vehicle fuel at the time required under s. 78.12(5) or who fraudulently withholds or appropriates or otherwise uses such moneys or any portion thereof belonging to this state is guilty of theft and shall be punished as provided by law for the crime of theft, irrespective of whether or not that licensee has or claims to have any interest in the moneys so received by that licensee.
Wis. Stat. § 78.15