private static function get_custom_schemas() {

		$quiz_fields = array();

		/**
		 * Handle old quiz layout compatibility API:
		 * Translate the old filter into the new one for quizzes.
		 */
		if ( get_theme_support( 'lifterlms-quizzes' ) && has_filter( 'llms_get_quiz_theme_settings' ) ) {

			$theme = wp_get_theme();

			$old = llms_get_quiz_theme_setting( 'layout' );

			$field = array(
				'attribute' => $old['id'],
				'id'        => $old['id'],
				'label'     => $old['name'],
				'type'      => ( 'select' === $old['type'] ) ? 'select' : 'radio',
				'options'   => $old['options'],
			);

			if ( isset( $old['id_prefix'] ) ) {
				$field['attribute_prefix'] = $old['id_prefix'];
			}

			$quiz_fields[ sprintf( '%s_backwards_theme_group', $theme->get_stylesheet() ) ] = array(
				// Translators: %s = Theme name.
				'title'      => sprintf( __( '%s Theme Settings', 'lifterlms' ), $theme->get( 'Name' ) ),
				'toggleable' => true,
				'fields'     => array( array( $field ) ),
			);

		}

		/**
		 * Add custom fields to the LifterLMS Builder.
		 *
		 * @since 3.17.0
		 *
		 * @link https://lifterlms.com/docs/course-builder-custom-fields-for-developers
		 *
		 * @param array[] $fields Array of post types containing arrays of custom field data.
		 */
		return apply_filters(
			'llms_builder_register_custom_fields',
			array(
				'lesson' => array(),
				'quiz'   => $quiz_fields,
			)
		);
	}