Mdbook preprocessor: how to apply classes to markdown items?

I'd like to be able to put markdown items within a <div class=...> container, or just to tag them with custom classes. To achieve that, I'm trying to update mdbook-classy to mdbook 0.5.4, but I know little about how mdbook works internally. I've succeeded to make it compile and work again, except for the <div class=...> added by this preprocessor around the element that follows. For example, {:.red big} You can write **Markdown** paragraphs should produce this (with or without the paragraph ...