def METHOD_NAME(theme_css_dir, data):
    file_manifest = create_css_file_manifest(theme_css_dir)
    if data.get("path"):
        data["path"] = os.path.join(theme_css_dir, str(data["path"]))
    form = file_manifest(data)
    if not form.is_valid():
        raise InvalidThemeManifest()
    return form.cleaned_data