Ayn Rand and a few others distinguished between subject and theme. Subject is what you story's about, so goofy adventures with dragons and magic. Theme is what it means, what do you have to say or prove about the human condition. Others have said that the climax is a final test of your theme...
And the scorpion replied to the frog, "I can't help it, I'm a scorpion." Why do they eat planets in the first place, and what happens if they don't eat the planet?
Structure (or form) is simply the way events and characters are organised. Every story has a structure, by definition.
The dominant form in Western storytelling is the called the linear or classical form divided into three acts, it's what people expect. Setup with inciting incident...