Creating a custom theme is easy! You can either upload a PDF from other resources and put it on the pages of your book or create it from the designer using back grounds and elements!
 

Once uploaded or you have edited the theme you want for your book, all you need to do is go to Save > Save as a Template 
 

Select Standard Template, and you can choose which side or page to save. 

After naming the template, click on Save.




 

If you need the template, you can find it from Layouts > My templates > will prompt to My Templates > click on Templates > scroll down to find your saved Template. 








You can then add the template to your desired pages later on, also you can still edit it change it and save it as a new template! Remember to do this on other page!