A topic is "trust". A controlling idea is "trust is what you give before you have grounds to". The difference is that the second can be disagreed with, and anything that cannot be disagreed with cannot be the spine of a story, because there is nothing for the events to prove.
It is derived, not decided in advance. Writers who choose a message first and build a story to deliver it produce work that reads as an argument with characters attached, which audiences detect immediately and resent. The reliable order is to write, find the claim your story is actually making, and then strengthen it.
For brand work this is the sharpest available tool for the question "what is this film really saying". A film with two controlling ideas says nothing; a film with one that contradicts the brand’s behaviour says something worse.
What is a controlling idea?
The claim a story makes about how the world works, stated as something that could be argued with. "Trust" is a topic; "trust is what you give before you have grounds to" is a controlling idea.
Should you decide the theme before writing?
Usually not. Deciding the message first produces work that reads as an argument with characters attached. Write, find the claim the story is actually making, then sharpen it.