If you use oStorybook as an advanced tool to structure your book and text but work with external software like LibreOffice, then this chapter does not concern you.
On the other hand, if you use oStorybook to enter your text you might want to give a more personal appearance to the result to be exported. You have already covered the subject in the chapter "11.4 Export and publish the book";here is a little more detail what you can do before and after.
A CSS, or Cascading Style Sheet in English, is a style sheet commonly used in web design, with files in HTML format. I recommend the very excellent Wikipedia article devoted to the subject. Beware: if you are not familiar with this, it will take you a long time to master this language, because it is above all a language.
So you have designed your own CSS, to use it you will have to indicate it in the export options as mentioned in chapter "11.2.4 HTML Options".
If not, there is a "standard" CSS that will be included in your exported HTML files.
If you are not a fan of computing and creating your own CSS bores you or is beyond your current ability, that does not matter as you can use LibreOffice to fine-tune your presentation.
Export your book in HTML format, with the option "one big file". You can then open the product file in LibreOffice and rework it as you wish. However, if you change the text in addition to formatting, you will need to make the same corrections in your oStorybook file.
See also some tips in chapter 17.