Source

File: wp-includes/general-template.php

function wp_admin_css_uri( $file = 'wp-admin' ) {
	if ( defined( 'WP_INSTALLING' ) ) {
		$_file = "./$file.css";
	} else {
		$_file = admin_url( "$file.css" );
	}
	$_file = add_query_arg( 'version', get_bloginfo( 'version' ), $_file );

	/**
	 * Filters the URI of a WordPress admin CSS file.
	 *
	 * @since 2.3.0
	 *
	 * @param string $_file Relative path to the file with query arguments attached.
	 * @param string $file  Relative path to the file, minus its ".css" extension.
	 */
	return apply_filters( 'wp_admin_css_uri', $_file, $file );
}