Can I Attach My Own Cascading Stylesheets?

Answer

Yes - you can just copy the stylesheet over the one created by Author-it.

However, the format of all the text elements is controlled by the Author-it Style objects, which are then published into a CSS file for all your HTML pages. So if you modify each Style object - on the HTML tab adjust the font, background, spacing etc - this will modify the CSS file when you republish your Book.