Retrieves the URL of a file in the theme.

Searches in the stylesheet directory before the template directory so themes
which inherit from a parent theme can just override one file.

function get_theme_file_uri( $file = '' ) {
	$file = ltrim( $file, '/' );

	if ( empty( $file ) ) {
		$url = get_stylesheet_directory_uri();
	} elseif ( file_exists( get_stylesheet_directory() . '/' . $file ) ) {
		$url = get_stylesheet_directory_uri() . '/' . $file;
	} else {
		$url = get_template_directory_uri() . '/' . $file;
	}

	/**
	 * Filters the URL to a file in the theme.
	 *
	 * @since 4.7.0
	 *
	 * @param string $url  The file URL.
	 * @param string $file The requested file to search for.
	 */
	return apply_filters( 'theme_file_uri', $url, $file );
}