This is the public side ("frontend") of your application.
Everything related to it is contained inside /frontend subdirectory.
You can treat this directory as a /protected subdirectory equivalent.
Frontend is crystal clear HTML5 Boilerplate.
It's expected that you are going to write your own 100% custom design anyway.
Points of interest:
/backend/components/FrontendController.php is the base for all frontend controllers.
It registers all required styles and scripts for common frontend UI.
Layout is /frontend/views/layouts/main.php.
Note that in this layout there is the Google Analytics code already inserted as a widget.
Just provide your GA ID in the ['params']['google.analytics.id'] section of a config,
by specifying it in the /frontend/config/environments/prod.php, for example.