Can a stroke or closed head injury directly affect your personality?

Yes. Particularly if there was damage to the frontal lobes.

Absolutely. One would think that in this modern age, people would have this basic understanding that the brain is the seat of the personality.

Yes. There are areas in brain that controls emotions, impulse, social appropriateness, temper - so people can have rage, impulse control issues, disinhibition, outbursts, difficulty coping.