In my opinion, the major theme of this speech is that slaves need to rebel against the system of slavery. Garnet did not necessarily mean that slaves should actually rebel violently, but he did advocate that they should resist their oppressors.
The point here is that Garnet is not telling slaves to make the best of their situation. He is also not telling them to wait until someone else comes to rescue them from slavery. In other words, he is not telling them to hope for abolitionists to get them freed. He is telling them to take it upon themselves to gain their freedom.
So I think what is distinctive about this speech, its main theme, is the idea that slaves themselves must get up and do something to free themselves.
Here is a quote to support this:
Let your motto be resistance! resistance! RESISTANCE! No oppressed people have ever secured their liberty without resistance.