Creates the theme package (.zip) from templates and optionally
    assets installed in the ``build_dir``.

def fill_package(app_name, build_dir=None, install_dir=None):
    """
    Creates the theme package (.zip) from templates and optionally
    assets installed in the ``build_dir``.
    """
    zip_path = os.path.join(install_dir, '%s.zip' % app_name)
    with zipfile.ZipFile(zip_path, 'w') as zip_file:
        fill_package_zip(zip_file, os.path.dirname(build_dir), prefix=app_name)
    return zip_path