I feel as if labeling themes is pretty standard across the board.
What is a theme: it is a core story question that the characters interact with. It is the big idea of the novel or an underlying message.
Examples: What is identity?
We often identify themes with large and grand sweeping questions and gestures: what is it to be evil? What is it to be human?
This is true, in the literary sense. However, I think all books touch on themes at least a little. It might not be trying to teach some grand gesture, but they do have themes. Which is why I break this down (for the rating system) into key words.
You can have Identity, Hope, Grey Morality (what is it to be evil?), Love, Acceptance. Or we can have Self Identity, Coming of Age, Questioning:Truth, Questioning: Evil, Saving the World, or even Defending Identity. Words and phrases that will reveal the theme without giving too much away and are still general.
These should be positive in general, or more positive. I would not expect for all themes to be positive themes, but this is going to be the section that shows the positives of the book, and how it is going to make the mind think, how this book is intended to leave an impact.
On the flip side we have
These are the warnings that are essentially the triggers for the book. These are NOT necessarily why the book is rated how it is (content rating) for that is based on a scale, but rather things that the author should know are in the book before reading. Character Death in a YA-D is different than character death in a YA-PW, but they are both character deaths. Destruction in a YA-M is different than a YA-E but it is still destruction. As such it will still be listed, it just won’t be expected to be as violent.
Trigger warnings are warnings to protect people from having a reaction to the content. This is a protection measure, so that a person knows whether to pick up a book or not based on their own personal interests. The intensity of the content in the book does inform the rating in a way, but to say that death can only be in YA-D or higher, is not fair, because all people should have the chance to touch and explore mature topics, what matters is how they are handled in the book.