All these embedded portlets are non-instanceable as defined in the
liferay-portlet.xml file. A non-instanceable portlet has only one instance on a portal page and therefore, its portlet ID is static. For instanceable portlets such as the Web Content Display portlet, the portlet ID is generated when an instance of the portlet is created, and therefore, can be unknown when the theme is created. This makes it more difficult to embed such portlets in a theme.
Custom portlets can also be embedded in a theme.
The first thing you need to know is the porlet ID. This can be found out by following these steps: