Add backend styles for gutenberg, where style-editor.css is not available from theme. (like with Oxygen)

plugin.php

add_action( 'enqueue_block_editor_assets', 'lit_add_gutenberg_styles' );
/**
 * Add backend styles for Gutenberg
 */
function lit_add_gutenberg_styles() {
	wp_enqueue_style( 'custom-gutenberg-editor-styles', plugin_dir_url( __FILE__ ) . '/editor-styles.css');
}