Am I sinful if someone else uses pornographic websites found on a phone I bought, knowing that I did not download them, and that it was stolen from me on the same day of purchase and never entered my home? And what is the way to repent and expiate for that?

The owner bears no sin if the thief uses his phone for an unlawful purpose, because the owner did not authorize that.

However, if a person buys a phone intending to use it for evil, he must repent of this intention and resolve, because the strength of intention and determination to commit evil is held accountable according to the verifying scholars, as stated in the noble hadith: "When two Muslims meet with their swords, the killer and the killed are in the Hellfire." Imam An-Nawawi clarified that Qadi Abu Bakr ibn At-Tayyib believes that whoever resolves to commit a sin in his heart is sinful for his belief and resolve. This is the view adopted by the majority of the Salaf and scholars.