The Fox uses a lot of Customization options to build up your theme style. So while using Fox theme, if you want to switch to a child theme, please following steps:
Step 1: While you still have Fox active, install this plugin. Go to Customize > Import/Export. Click “Export” you’ll get a file fox-export.dat saved to your computer.
Step 2: Activate your Fox child theme. Go to Customize > Import/Export again. Upload your fox-export.dat above, check to “Download and import image files?” and hit “Import“.
Now you have child theme activated with same style as your original Fox theme.