"Is this grounds for a mistrial?
Well surely the Valentine's T shirts should be."
You can't have a mistrial after the verdict is read. The trial phase is over, now comes the sentencing phase, then the move for a new trial (which will be denied) then the appeal (anything can happen).
Can you have jury nullification at this point? Can't the judge say, based on the statements made by the journo juror, that the jury based their verdict on something other than the evidence given to them?