Article/video response(analysis): 

  Theme, oh the use of them. It is usually a essential asset to a story. When I was little I understood it but didn't at the same time. I thought it only required a vague generalization for the story's "moral of the story". Oh how I was wrong. A theme is a more related, detailed analysis of what the story's lesson is whether it being a show, movie, book etc. For instance, when you look at the "Return of the Jedi", you don't just say "good conquers evil"; NO THAT'S TOO VAGUE, you should probably say something like "Though one has been corrupted to their very soul, they always have a chance to redeem themselves". People always thought the movie was about Luke's return but when you actually look at it, it was about his father Darth Vader. Of course the only way to truly see the them is to study the film. Also to study the protagonist's journey and see how it molds them. Looking at that analysis and compare it to the movie, it makes more sense than just saying the generic "good conquers all".

  Themes can be thought of before the creation of the story or just materialize before the writer while creating his/her project. Either one works depending on the person creating the story. There is no rule book on how to create a theme, it just happens. It can be called a loose theme. From personal experience, its good (for me) to have a rough idea but to draw in the details later. Just so I can be organized in some way. You don't need to but that's my personal recommendation.
  It's hard to imagine a movie or any type of media without some sort of theme. Kids movies have upbeat motivational themes and adult films tend to have realistic themes that tend to connect sometimes almost on a spiritual level. Themes tend to show a comparison to what the writers state of mind is in. Sorta subliminal subconscious message that wants to get out. It's interesting because you can see it a lot within the writers work.