From Joomla! Documentation
Template style is a feature introduced in that allows users to assign different template styles to individual menu items. By default, Joomla assigns a template style to all menu items upon installation. A yellow star indicates the default template style in use. A default template style can be partially or completely overridden by assigning different template styles to the desired menu items in order to obtain a different look for their respective pages.
A template style can be assigned to menu items one of two ways.
- Template manager Extensions → Templates → Styles
- Editing a menu item under Menus → [Menu Name] → [Menu item]
See also: How to assign more than one template to the Frontend