After installing the Beta update today, when I visit a document that I’d formerly set the text background dark for, it reverts to white text background, and even after updating the background again, it turns white at next doc visit. But the “margins” retain their color setting (e.g. the margins set by the ruler, where the LHS looks jagged).
And then I run into an issue when editing text background for the whole document. First I have to select all, and that highlights the text darkish blue which masks the “live preview” of changing color, so I can’t see the target background color much less see it with respect to the actual text foreground color.
Finally it’s still very confusing to figure out an approach to set a background color for the document (e.g. the ‘margins’ and the text background work differently and I always have to try to remember where which is set, from the font panel or document editing bar).
Are you using the dark mode color scheme in Mojave?
Correct: Using dark mode under latest Mojave update.
Version 10.14.6
The appearance of rich text is now adapted to Dark mode, the next beta will improve this. Therefore I’d suggest to remove the text background color as e.g. white/grey/black backgrounds are automatically adapted.
Thanks. I just figured that out by visiting a document I hadn’t changed the background information for. Well that’s pretty cool. Progress 
Thanks!
I’m still trying to figure out how to ‘remove’ a background color, since the only options seem to be to simply change the existing.
Put the cursor in a document and press Command-A to Select All. Then press Command-T to open the font panel where you can change the font and background colors.
Unfortunately, having previously set the background color, that merely changes it, doesn’t remove it, so apparently the default dark mode behavior doesn’t kick-in like it does for documents I’d never changed the background for. And if I try to copy the carefully formatted document to another RTF document it carries those ‘tampered with’ background attributes with it, so I’m still in the same boat. The problem is it took me an hour or so to adapt that page from a manual and so I can’t really start over with it.
Setting the colors dynamically seems to be interacting oddly with the default dark mode with macOS. For example, I just did select all and then moved the font panel background slider to black, and this is what I get:
And after all that still have the problem that it forgets everything I did when I revisit the doc:
Except that background looks totally white on my screen but the screenshot looks gray in the upload to that last post
I just went to a page that never had any color editing and it showed decent defaults for dark mode: Black background, white text. I found that a bit too harsh so I selected all the text and tried to darken the color, but that was also confusing because the color selector showed the text (that has been inverted to look white on the screen) was actually black, so it was unclear how to change it, and because everything is selected, similarly to what I mentioned about the background color, above, I couldn’t actually tell what color I was changing everything to until I deselected everything.
