If a user styles the text in the sampler, the CSS gets generated automatically and is included in the compiled ebook (subject to ebook limitations). Sample project attached, if it helps to have a sandbox.
That all works for as long as you do not compile for Kindle KF8/Mobi Ebook as once you select that as for your compile format, the formatting window has no effect and does not seem to work. Right? If I select the 3Pub3 format and then run that export file through Kindle Previewer 3, I am able to make some adjustments that end up in the mobi file. However, if I select Kindle KF8/Mobi as compile format, whatever I change in the CSS does not seem to end up in the book.
No, that is not right. Bridey's Scrivener Test.scriv project is already designed EPub3/KF8 — his TOC document uses the Table of Contents Section Layout which uses Heading 1 style for title and toc style for text. He has modified Heading 1 to be green using the Styles panel, and made a toc style that is magenta text, also edited using Styles and adding a CSS class called toc, but he has added no custom CSS, he has only used the GUI to make these changes AFAICT.
As I mentioned above (viewtopic.php?p=256588#p256588), you can modify font size, line spacing, bold, italic, color, indentation, margins using the Styles visual editor. If you have more detailed changes, then the CSS can be edited for more specific needs. Again this is for EPub3/KF8, where the output is semantic clean and elegant HTML5.
Actually I may have misunderstood you. You are saying EPub3 looks OK, and if you maually convert EPub3 to KF8 it is OK, but if you use KF8 directly then you can't see the changes? This could be possible as the base CSS is different, I'll need to test.
EDIT: KF8 output is working as EPub3 for me, here i've centered the heading, coloured it green, bold+italics and the text is right-aligned, bolded coloured red using the Styles editor, viewed in Calibre's .mobi viewer:
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