Annie and Les, i am SO sorry you spent so much of your time and energy on this! but thank you for not giving up and sticking to the end of the issue.
No problem Isabelle, glad you got it back to transparent again!
Please remember, if not necessary, do not make it more complicated.
Just set the default in customizer ( as you did ) and do not start changing settings per page too ( unless you really need/want to have different styling on that page ).
Also, learn how to migrate a backup ( exact copy of your site ) into a subfolder on your domein ( like for example: https://oilpaintingbyisabelle.com/test
That way you can experiment and learn there, without messing up the live site.
It is also great to try out important big updates, to see if all works well after it.
I usally have this kind a staging site for all clients. If all goes well there, I then go and update in their live site or shop.
Keep up the good work!
Annie