With Vers. 3, I can’t seem to change the colour of the editor behind the text anymore. Here’s what happens when I change the editor background colour. It changes as I ask, except behind the text. Have I missed some step?
Do you have some sort of highlighting applied to the text in question?
No highlight. That’s just me typing in the editor. Happens in new documents too.
That does definitely look like you have a white highlight applied to your text—perhaps even as a default. I’d select that text with ⌘A and use the Format ▸ Highlight ▸ Remove Color menu command, and if that does the trick, go into the Editing: Formatting preference pane and do the same thing there to the sample text.
I’m with you up to the last part. The first part of that did work. And new documents seem okay now. But all the other documents still have this strange formatting and I just don’t know how I stumbled into it.
I guess what I’m not getting is how is it possible to have all the text highlighted “white” when white isn’t one of the highlight colour options.
If you’ve removed the strange highlight in Preferences > Editing > Formatting, then it will only affect new documents (it doesn’t retroactive change existing ones).
Easiest way to remove the oddness from the existing documents is to highlight them all in the binder, toggle cmd-2 till you’re in Scrivenings mode, cmd-a to select everything, then Format ▸ Highlight ▸ Remove Colour again.
HTH
As for how it got that way, most commonly it comes from having pasted content from a website. Try it from this post for example, and you’ll get a greyish background in the editor. With a perfectly white background you may never notice it though until you change the editor colour. 
It could have then gotten into preferences by using one of these paragraphs to set the defaults.
Okay. Many thanks, all.