Contents here go into a folder named app-extend inside the front end folder, next to app-base.
The files here will be compiled along the base system through brunch into the single page application and allow for customizing not only the style but also the system itself.
Styling
Any style sheets inside css/ will be concatenated and included into the base application after the default style for them to be able to override these.
Any templates inside views/templates/ will be compiled and added to the system. If the paths (mind the folder structure) already exist there they will override the default templates.
Assets
Anything inside assets/ gets copied along into the public folder. Add static content like images here but remember to keep it slim.
Cells and other system items
You can create your own views / models / ... here or override the default ones. Examples will follow soon.
Modules
Inside modules/ there is an example iframe module that shows how to connect with the system dispatcher. To test include two of these modules into an iframe cell inside a set. Click the ping/pong buttons in one iframe and see the messages arrive at the other one.
Note that it is not necessary to have the modules be included here into the app-extend folder they can live somewhere else ... even on a separate server / domain.