After creating a theme on the Formatting Tab, you can use this theme across any book in Atticus. To do this, you'll first want to create the next book in Atticus, then proceed to the Formatting Tab. There, you can access your saved themes from the carousel at the top of the page. Just select the theme that you would like to use and all of the theme settings will be applied to the book.
You can also mark frequently used themes as favorites by clicking the heart icon located near the bottom right corner of the theme preview. Favorited themes will appear at the start of your theme list for easy access.