Hello Pablo,
Thanks for writing in!
It looks like you havent activated child theme on your website. Did you customized any files of parent X theme? (Main theme) If you have done modifications to the core theme files and are not using a child theme then updating would overwrite all those changes. So we advise you to edit theme files into child theme as you don’t need to update the child theme. As far as custom css you’ve put in the Customizer: this won’t be affected at all. However I recommend doing a backup of the whole site + the Customizer settings.
Before initiating upgrade process, we recommend you to please walk-through our theme and plugin update guide:
https://community.theme.co/kb/updating-your-theme-and-plugins/
Thanks.