Is the existence of the button even related to build? Why is this so mysterious? If you have the black theme on, and Win10 in dark mode, it's absolutely ridiculous to have a white background reading pane in Outlook.
I'm beginning to wonder the same thing. I think the change has been pushed out to my machine, but something is stopping it from turning on.
Here's a possible x-factor... My company hasn't pushed out Windows 10 major updates yet, so I'm still running 1803... so I don't have system-wide dark mode on Windows 10, which I believe was a 1809 change.
While I believe it's ridiculous to be able to enable dark mode on Office generally, but only have the reading pane be dark if system-wide dark mode is enabled, I'm wondering whether there's a conditional buried in there somewhere that is preventing it from
coming on.
Just a thought, since at the moment I'm scratching my head too.