Themes are special XML files introduced as of 0.5.0 that describe a configuration environment portably. They are described more thoroughly in their respective section in this document. However, for now, it is suffice to say that themes can be loaded via a similar directive to the import statement:
theme themes/wetlands.xml
or
theme /home/mgilfix/peep/peep-0.5.x/themes/wetlands.xml