You can customize many elements of your theme by changing the settings in your theme options.
Go to Appearance Theme Options from the left sidebar menu of your dashboard. This will bring you to your Global theme options. You'll also see three other tabs at the top for access to your Web Options, PDF Options, and Ebook Options.
Global Options apply to all output formats across the board. Changes here will affect your web, PDF, and ebook exports.
Web Options apply only to your Pressbooks webbook.
PDF Options will apply only to your print and digital PDF exports.
Ebook Options apply only to your EPUB exports.
Other customizations can be done with CSS. Check out our user guide for more information: https://guide.pressbooks.com/chapter/customizing-your-exports-with-custom-styles/
Note that Pressbooks cannot offer comprehensive technical support for customizations to users' CSS.