Added WP Gutenberg custom stylesheet support

vue-theme/functions.php

 // Add featured image support
 function craft_post_thumbnails() { add_theme_support( 'post-thumbnails' ); }
 add_action( 'after_setup_theme', 'craft_post_thumbnails' );
+
+// Gutenberg custom stylesheet for editor
+add_theme_support('editor-styles');
+add_editor_style( 'editor-style.css' );