I have a custom stylesheet for MD files set in DT’s preferences. That works fine in DT itself. However, when I convert this MD file to HTML (using either the context menu or the main menu), the HTML file does not contain the style sheet nor a reference to it.
Whereas the converted HTML includes the JavaScript and CSS files for Prism…
I think that conversion to HTML should incorporate the styles from the user-defined stylesheet (obviously not a reference to it, because that would in most cases result in a non-portable HTML file).
Over here a reference is indeed added. How does the preference look like?
Wow, you’re fast.
It’s a devonthink-link: x-devonthink-item://0841105D-70B7-4518-8E2C-68E25CF8FC38
And that reference is definitely not inside the HTML source?
Blush. In fact, it is 
in a link element. I was looking for style. But may I suggest to actually copy the CSS into the HTML file instead of referencing it? A x-devonthink-URL is meaningful only on the machine where DT is running, I suppose.
File > Export > as Website… should resolve the link if the linked item is exported too.
Thanks. Still: If prism.js/prism.css are included, a custom stylesheet should be too in my opinion. Especially since the prism stuff is readily available via CDN or elsewhere.