Visually distinguish definitions, theorems, etc. from normal text

I'm writing an HTML document where I include definitions, theorems and the like using divs. However, it's difficult to distinguish the text of e.g. a theorem from the normal text in the paragraph that follows it.

Is there a simple way to format divs of definitions, theorems, etc. such that it's easy to visually separate them from the rest of the document?

I would recommend to use CSS to modify those content. You should inspect HTML and see if Quarto does apply some class on them. Otherwise, you could also wrap them in Callouts for example.

With HTML format, CSS will be the way to customize.