Greetings! I note there are help articles regarding child themes:
https://theme.co/apex/forum/t/setup-how-to-setup-child-themes/57, and a follow-up comment in the forum:
This website was apparently never set up on a child theme, and presumably has experienced significant parent theme changes over the years. So the challenge is evident: how best to transition to a child theme at this stage, if at all possible.
I look forward to your recommendations as to next steps.
Dog Fence Ontario Support