[Quoth]

The LLM like GPT are parasites copying other's work and often producing plausible rubbish. IMO only suitable for producing Advertising copy that's then checked by a human.

Crazy to generate CSS or code..

Also there are many situations were simply appending CSS won't fix things.

Line-height can be removed entirely by caliber automatically and isn't needed at all in an ebook.

Re: font size, font family, margins
You need to look at the existing HTML and CSS and see why they have done what ever it is. You need to identify which is the body paragraph, first paragraph and headings/chapters as a minimum. One mad looking ebook turned out to have no paragraphs or headings. Everything was <div class="whatever">some text</div>. The actual CSS had unlikely stuff too.