In my page about Word templates I say:
The idea of these is that layout.css defines the appearance of the elements of the page that are not coming from the content such as the TOC, the Index, headers and footers and so on. Then content.css provides a single CSS for the content. As the print outputs are a single document, the topics must all have the same CSS applied.
What that means is in the sections of the document that are not your topics, for example your TOC, those are the styles that need to be defined in llayout.css.
Later I say:
If your topics only reference one CSS throughout the project, you do not need a CSS in the page layout for the content, only the elements of the layout itself need to be defined.
If different topics reference more than one CSS, you need to define a single CSS for the content in print outputs, as explained above. That CSS needs to be the Stylesheets group in the template, not elsewhere in your project.
That is pretty clear and I think you understood that bit. In addition, you might have defined one CSS for online but want something different in print. You could define that here. Maybe just copy in the online CSS and edit that as required for print.
If you want and only one css is used in topics, content.css can be deleted.
Does that clarify it and work for you?
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My site www.grainge.org includes many free Authoring and RoboHelp resources that may be of help.