Each Theme subsection includes several properties whose values are referenced by one or more Style properties. The property values in Theme subsections are set when you choose a Theme. You may modify the properties if desired.
The Theme.Headings section includes style properties that affect site headings. Headings are text components that are used to divide page content into sections. Headings are distinct from the Header which is a block of content at the top of the page.
See the Styles Section help page for more details.