Theft is often discussed as a single missing object, but contemporary disputes may involve intellectual property, unauthorized accounts, identity misuse, unpaid labor, diverted revenue, or someone claiming credit for another person's work.
Good documentation is boring until it becomes priceless. Save contracts, drafts, timestamps, receipts, account records, and communication.
Publicly calling conduct theft is a factual allegation. Use precise language, consult counsel where appropriate, and distinguish a documented event from a disputed interpretation.