Steps to reproduce:
1. Create and install an empty parent-theme (mvn install)
Result: ~/.m2/repository/com/liferay/themes/parent-theme/1.0-SNAPSHOT/parent-theme-1.0-SNAPSHOT.war with size 784K
2. Create a child-theme selecting as parent the previous theme:
3. Generate the artifact child-theme-1.0-SNAPSHOT.war running "mvn install"
Result: ~/.m2/repository/com/liferay/themes/child-theme/1.0-SNAPSHOT/child-theme-1.0-SNAPSHOT.war with size 152 MB
Taking a look into the war, a folder named appServerPortalDir/ with all the portal libraries is included.