The theme that the texts share is "Equality is worth fighting for."
Both the Gettysburg Address and Martin Luther King Jr.'s "Letter from Birmingham Jail" emphasize the importance of equality and the struggle for civil rights. Lincoln speaks about the nation being dedicated to the proposition that all men are created equal, while King argues for the urgent need to fight against injustices affecting African Americans. Therefore, the shared theme focuses on the value of equality and the inherent struggle involved in achieving it.