Hello,
Customization of theme files means that you changed code of any file in wp-content/themes/xstore folder. If you do not change anything there then you won’t lose it. You won’t lose products, pages, anything you created from WP Dashboard. Anyway, do the backup of your files and DB just in case. What theme version are you using now? Do you use child theme? Did you change anything in child theme files if you use it?
Regards