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.Sans-Serif Text section includes style properties that affect the text that appears in parts of the site where GedSite presumes that a sans-serif font-family is a better choice than a serif font-family. To insulate those parts of the site from the font-family choice made in the Body Text section, several styles default to the Sans-Serif Text properties.
See the Styles Section help page for more details.