Hi There,
Thanks for writing in! if you have the latest X|Theme but using the old child theme then you need to update the @import statement on your child theme.
If you don’t have any custom template of your child theme but only custom CSS on the style.css file. Then you can simple install the new child theme and copy all the custom CSS on your old child theme.
Please follow this post for more details.
Let us know how it goes.
Cheers!