editing page background color was lost in transfer

I should never have doubted you. Of course, it would have helped if I'd caught that you were erasing the color value rather than entering a transparency value, which is all I'd ever done. I must have stopped and started that video 10 times.
I'd guess that my old theme represented transparency differently, if it weren't for the fact that Tom got it to work in the new theme without changing the content color value, only the body value. And now, of course, I'm not sure why, after working solidly after Tom's fix, it didn't after last week's transfer when, again, I hadn't been into the customizer to make any changes to the content, body, or anywhere else.
But it works now. And I thank you not only for the fix but for your patience.
Til next time.
Laura Stella Sitges

No problem!

The way the block editor works changes from time to time and this is the best way for now without adding any CSS :)

By block editor, are you referring to WP's Gutenberg? I don't know of another.

Yes :)