I think one of the major themes that Douglass is trying to illustrate is that Ignorance is used as a means to perpetuate slavery. You can read about it at the GradeSaver link below. There are many things Douglass relates about this time period. There is the historical context of slavery. He relates Christianity's link with slavery in both a good context as well as the bad. He relates the number of slaves on plantations as well as the haunting melodies of the slaves as they traversed the forests and worked the fields.
http://www.gradesaver.com/narrative-of-the-life-of-frederick-douglass-an-american-slave-written-by-himself/study-guide/themes
