This is why we specifically put DO NOT EDIT on XenBase and the styles themselves and all edits should go inside only your child. Literally do not edit XenBase DO NOT EDIT or Fusion Gamer DO NOT EDIT.
To fix this:
Copy all of your extra.css on your "Gamer" style and paste it inside like a notepad file to copy it.
Next revert the extra.css template on your Gamer Style.
Next go to extra.css in Fusion Gamer DO NOT EDIT style, revert extra.css
Finally go to extra.css in XenBase DO NOT EDIT style, revert extra.css.
Now go back to your "Gamer" style, paste the CSS back inside EXTRA.CSS, going forward don't edit XenBase/Fusion Gamer DO NOT EDIT.