I am going to assume that a "theme statement" is the same as a thesis statement, a statement that you can explain and support with evidence from a piece of literature. For your assignment over Twelve Angry Men, consider the events in the play and the characters. What do you believe to be true about them?
For instance, based on the evidence presented at trial, was the jury's verdict fair and just? The play is developed to suggest that the jury did reach a fair verdict when they found the boy not guilty. From this idea, you could write this as your theme statement: In Twelve Angry Men, the jury reached a fair verdict. Your job then would be to explain why this is true and to back up your discussion with examples and perhaps quotations from the play.
There are many statements you could make about the play and its characters that could be explained and supported with evidence from the play. For instance, are these true statements that you could explain and back up with facts?
Without Juror Number 8, the boy would have been unfairly convicted.
Some of the jurors showed irresponsible behavior.
The boy's attorney did not do a good job in defending him in court.
Those are just some suggestions to get you started. Again, begin by asking yourself, "What do I think is true about this play or the characters in it?" Good luck!