I tried to tweak the style settings even more, but really messed it up and now can’t see the cursor position, read the text on the current line, or the anything in the selected text. And now NotePad++ is impossible for me to use any more. How do I reset Dark Mode’s settings to their ‘factory’ settings?
If there is a link or button to do this on the style dialogs areas or the built-in Help screen, I haven’t found it?

There is no button. People wanting a “Factory reset” is a rare thing (you’re the first that I’ve ever noticed asking for it… though I can see why it would be nice)

Would reinstalling NotePad++ fix this?

No, because reinstalling Notepad++ doesn’t change the files that are in %AppData%\Notepad++, where your customizations are stored. (assuming normal installation)

But the good news is, that’s the solution, too. When you customize the Dark Mode stylings, they end up in %AppData%\Notepad++\themes\ as *.xml. So if you exit Notepad++, then use Windows Explorer and paste in that directory (don’t worry, Windows knows to do “the right thing” with the %AppData% portion, so you can literally copy the entire %AppData%\Notepad++\themes\ and paste it in the Explorer location bar), then you can delete DarkModeDefault.xml from that directory (and any others in there that you want to reset). When you run Notepad++ again, it will use the copy of DarkModeDefault that ships with Notepad++, and if you make any customizations, it will save a new copy in your AppData location.

(If you have a non-normal installation, things will have to be different, and you’ll have to share your ?-menu’s Debug Info with us for us to be able to tell you where the files will be.)

Oh, and if after this issue is solved, how do I set the general screen background to the the darkest black rather than the sort of gray that it was at when I chose Dark Mode, before I wrecked that?

Settings > Style Configurator > Language: Global Styles > Style: Default Style, then click on the background color box, and select the true black: