A Pre-Designed Trope System is a system of tropes the author set into play consistently. In this case, it's how tropes in the story are used to construct, de-construct, subvert the central tropesthemes. Notice the plural.
This type of trope framework gives the story a distinct flavor and makes the story easy to grasp, since the ideas are organized.
You know you have the central tropes when you can use other tropes in the story point to it.
The examples should include how the other tropes revolve around the central tropes, or how certain tropes were on purpose not evoked because of limiting vision of the central themes.
Use this page to help you find the central tropes: Sum This Up In One Trope