Oh! I understand better! It will help me to avoid that I lose all my changes. Thank you for have pointed me out, Russ. If you will not tell me this, I would have probably lost everything.No, you missed step number 4.
Ideally you want all of your changes in a child of the core style. So when you go to upgrade you simply import XenBase, then Core. Then all the updates trickle down to your custom style without losing any customization.
I know you've done quite a few edits, but I would port the changes you have done so far over to a custom child style.