Source

File: wp-includes/class-wp-customize-setting.php

protected function get_root_value( $default = null ) {
		$id_base = $this->id_data['base'];
		if ( 'option' === $this->type ) {
			return get_option( $id_base, $default );
		} elseif ( 'theme_mod' === $this->type ) {
			return get_theme_mod( $id_base, $default );
		} else {
			/*
			 * Any WP_Customize_Setting subclass implementing aggregate multidimensional
			 * will need to override this method to obtain the data from the appropriate
			 * location.
			 */
			return $default;
		}
	}