Let my start by saying that you determine the theme of a text.
We can sit in class day-in and day-out, and I can tell you the themes I see; I can even prove to you that the themes I see appear in our readings. But when you are reading, if you relate the text to your life, you'll see the theme you need to see. Then it is up to you to prove to me and your classmates that that theme exists.
So, again, you determine the theme of a text. But, you have to prove what you see is correct using the literary terms we use in class.
Theme is a unifying or dominant idea, lesson, or reflection on life that appears in a text.
The video above does a decent job at introducing you to
some of the thematic concepts we will see in class.