I cannot recall how I previously got a child theme file to upload and appear on Appearance > Themes so it could be activated. I had no problem uploading the Weaver Xtreme. I tried posting the contents of the child theme style.css file here but got an unsafe warning that deleted my entire post. Thank you.
I will let @weaver answer on how to make a child theme appear in Appearance > Theme.
But if all you need is custom CSS, there is no need for a child theme.
You can place all your custom CSS In the theme Global Custom CSS rule box, either through the Customizer, or Xtreme Admin > Main Options > Fonts & Custom > Custom CSS Rule box
When I first started 15 years ago, I was told to always activate a child theme in front of the theme of choice to preserve all changes and settings. Of course I make backups but I have always inserted a child theme. I just don’t remember how I got it loaded to Appearance > Themes. I have the data for the child theme in a style.css file, just can’t recall how to get it loaded so I can activate it.
Weaver saves all the custom CSS with the theme settings so they are preserved during updates or deactivation activation.
You only need a child theme if you need to make PHP changes in the theme templates or function file.
If all you were using the child theme for was custom CSS, just paste all your custom CSS in the theme global Custom CSS Rule box as explained above and be done with it 🙂