A theme DOES NOT control the placement or behavior of fields or objects on a layout. When you create a new layout you also specify a theme to be used on that layout. The theme that is used on a layout can be changed anytime from the Layout menu in Layout mode. When you add an object to a layout, say a field, that field is formatted using the default style for that object as specified by the layout theme. You can check what theme is being used on a given layout by viewing the top of the Appearance tab in the Inspector.