Hello,
First of all, you need to backup all your files, DB and do Theme Options export before any update. Then you’ll be able to restore your site in case something goes wrong during the update.
As a rule, you should not lose any data or custom CSS after update. Only if you made changes in template files instead of child theme then these changes will be lost. That’s why we always recommend to use child theme for all the types of customization.
https://www.8theme.com/demo/docs/xstore/#installation_upgrade_23_theme_upgrade
Regards