private function theme_locations() {

		if ( isset( $this->theme_locations ) ) {
			return $this->theme_locations;
		}

		/**
		 * Filters the theme locations.
		 *
		 * @since 1.0.0
		 *
		 * @param string|string[] $theme_locations One or more theme locations.
		 */
		$theme_locations = (array) apply_filters( self::FILTER_THEME_LOCATIONS, [] );

		$this->theme_locations = array_map( 'strval', $theme_locations );

		return $this->theme_locations;
	}