In the backstage > Options > Preferences > Legend Window > Default text fonts , color and size can be defined for Title Pattern , Subtitle Pattern and Style Name Pattern.
All these default settings are ignored when creating a Thematic Map Legend .
These preferences, set from the backstage only apply to non-thematic legends.
All the thematic legend information comes from the Thematic templates in the installed directory. The thematic legends are created when the thematic layer is created, and this depends on which theme template is used. So the thematic legends get generated based on the thematic template, and the cartographic legends get generated using the information from the Legend Preferences window in the backstage.
This is a very useful tip to keep in mind.