Add editor styles which get autoprefixed with .editor-styles-wrapper

functions.php

function gutenberg_editor_style_setup() {
	  
	// Add support for editor styles
	  add_theme_support( 'editor-styles' );
  
	// Enqueue editor styles
	add_editor_style( "/css/gutenberg-style.css" );
}
add_action( 'after_setup_theme', 'gutenberg_editor_style_setup' );