You've already seen them throughout the new Joomla! backend: tabbed screens. The new tabbed Template Manager allows you to jump from the Styles tab to the Templates tab. Let's find out what those screens have to offer you.
The main purpose of the Styles screen is to allow you to select which of the available templates you want to use. As you can see, the current default frontend template is called Beez 2.0 (referred to as Beez2 in this screen). The default backend template is called Bluestork:
To change the default template, select the desired template and click on Make Default. To assign a template to the backend, select any of the Administrator templates and click on Make Default. In the Assigned column, you can see whether a template is in use (even if it isn't the default one), because it's assigned to one or more menu items.