In earlier versions WebHelp had Skins and Responsive HTML5 Help had layouts. The terminology has now changed. Layouts refers to the underlying functionality, the foundations. The term Skins will now be used for both outputs and both can be edited in the new Skin Editor.
Click Output > Output Settings and then click Skins. When you select what you want to edit, a preview will display on the left along with the Skin Components you can customize. As you click each component, its properties will be displayed on the right for editing.