A thematic biography is not merely a biography, which is an account of the events of a particular person's life. One could easily write a biography without ever engaging in the more intimate, more interesting questions a thematic biography can entail.
A thematic biography is a description and recounting of the life of the subject, but with an overarching emphasis on some other issue. The concept of a thematic biography is a type of writing that doesn't stick to facts alone, but seeks to incorporate facts in proving a predetermined point of view.
A thematic biography is the recounting of the historical events of a person's life from a definite point of view that usually changes the traditional interpretations of how that life is seen.
If you were to write a thematic biography on Abraham Lincoln, you would be overwhelmed by the choices you would have to make a cohesive thematic biography. That's because Lincoln's life touched on so many different political, social, and personal issues, so the choice of themes is pretty much limitless.
Why was Lincoln assassinated? John Wilkes Booth, a popular actor of the day, shot Lincoln in a Washington, DC theater. Booth allegedly did so because of Lincoln winning the Civil War against the South, and Booth, a southerner, was in anguish at the pain, confusion, and loss to the states he loved so dearly.
These facts are not a thematic biography. In order to write a thematic biography, you'd need to come up with a central thesis. In using the example of Lincoln's assassination, one could look at that fact several ways. One theme might be that Lincoln, by his own actions, brought the assassination on himself by how he allegedly humiliated the South during and after the Civil War.
If this were the central theme for the biography, the writer would purposely emphasis certain facts of Lincoln's actions and deeds that would prove that thesis.
If, on the other hand, the central thesis is that Lincoln's behavior and how he ran the Civil War could not have been any different if the war was to be won by the Union Army, and thus, there would've been no way to stop Booth from following through with the assassination, one would emphasis certain facts in a different way.
A thematic biography presents a case, almost as if it were a legal case, in a way that proves the contention of the author, as expressed in the central thesis of the writing.
The question remains if such a thematic biography can be considered to be nonfiction. Pundits who are opposed to the themes proven or disproven by a particular work might call the author's contentions "fiction," but that wouldn't necessarily be true.