Source

File: wp-includes/general-template.php

function wp_admin_css( $file = 'wp-admin', $force_echo = false ) {
	// For backward compatibility.
	$handle = 0 === strpos( $file, 'css/' ) ? substr( $file, 4 ) : $file;

	if ( wp_styles()->query( $handle ) ) {
		if ( $force_echo || did_action( 'wp_print_styles' ) ) {
			// We already printed the style queue. Print this one immediately.
			wp_print_styles( $handle );
		} else {
			// Add to style queue.
			wp_enqueue_style( $handle );
		}
		return;
	}

	$stylesheet_link = sprintf(
		"<link rel='stylesheet' href='%s' type='text/css' />\n",
		esc_url( wp_admin_css_uri( $file ) )
	);

	/**
	 * Filters the stylesheet link to the specified CSS file.
	 *
	 * If the site is set to display right-to-left, the RTL stylesheet link
	 * will be used instead.
	 *
	 * @since 2.3.0
	 * @param string $stylesheet_link HTML link element for the stylesheet.
	 * @param string $file            Style handle name or filename (without ".css" extension)
	 *                                relative to wp-admin/. Defaults to 'wp-admin'.
	 */
	echo apply_filters( 'wp_admin_css', $stylesheet_link, $file );

	if ( function_exists( 'is_rtl' ) && is_rtl() ) {
		$rtl_stylesheet_link = sprintf(
			"<link rel='stylesheet' href='%s' type='text/css' />\n",
			esc_url( wp_admin_css_uri( "$file-rtl" ) )
		);

		/** This filter is documented in wp-includes/general-template.php */
		echo apply_filters( 'wp_admin_css', $rtl_stylesheet_link, "$file-rtl" );
	}
}