A theme is a central and/or recurring idea. I like to think of theme as the glue that binds a story or work together. Without a central idea to any story, all we have is empty entertainment. Even if you take any seemingly basic story, like a Pixar movie that you’ve seen, you should be able to identify a number of themes.
Interpreting and determining ideas in texts is an essential life-skill that transcends just literary analysis. Ideas have the power to persuade, inspire, influence, subject, divide and conquer. This is why we need to have the critical thinking skills to determine what a text is saying explicitly, as well as implicitly (i.e. on the surface and more deeply).
Your interpretation and analysis of themes will be highly influenced by your existing beliefs, opinions and experiences. The wonderful (and for you, often frustrating) thing about literary analysis is that there is no one right answer; I argue that you can propose that a text is about whatever you want. However, and this is an important proviso, you must have the evidence to support your ideas and be able to convincingly incorporate that evidence in your arguments.
This is why I do not want you to be reading or searching online materials and typing into google phrases like “what are the themes of…?” Knowing the themes of the text you’re studying is probably not going to be important for your life; what is important is developing the skills to analyse the text and formulate your own ideas!