Leap Web is based on Engines. All things in app will overwrite the default behaviour. You can either create a new rails app and include the leap_web gem or clone the leap web repository and add your customizations to the app directory.
We use scss. It's a superset of css3. Add your customizations to app/assets/stylesheets.
If you have no use for one of the engines you can remove it from the Gemfile. Not however that your app might still need to provide some functionality for the other engines to work. For example the users engine provides current_user and other methods.