Is a person considered a thief if he secretly destroys his colleague's phone in retaliation for the colleague destroying his personal computer, and must he repent despite feeling wronged?

Your colleague's transgression against your device and its destruction is unlawful, and he is liable for whatever he damaged. However, it is not permissible for you to take his phone and throw it in the bathroom, because that is intentionally destroying respected property and squandering wealth, which Allah and His Messenger disliked, as stated in the Hadith: "Indeed, Allah dislikes for you three things: gossiping, squandering wealth, and excessive questioning." In addition, there is lying, which is not a characteristic of a believer.

The path to repentance is through seeking forgiveness and remorse, returning the value of the phone to its owner, and demanding from him the value of what he damaged in your device. If he denies your right, you may offset what is owed to him against your right. As for any amount exceeding your right, you are liable to compensate him for it.