Ok I found a way, sorry for the unnecessary question.
Just in case anyone might stumble on this question in the future, you can use frameworks by adding their link reference to the published html file. You just need to open it with a text editor and add to the first section the link ref (search for the title of the story and put it after it, if you're having doubts). For W3.CSS for example is this line:
<link rel="stylesheet" href="https://www.w3schools.com/w3css/4/w3.css">