I have updated to Android 9 pie. I have tried to set my Note9 to the Dark mode. However, I keep getting the message that I cannot set to the dark mode while using a theme. Is there anyway to not have a theme?
Open Samsung Theme's App > click Theme's at the bottom > at the top of the page you should see a section called 'My Theme's' > click 'View All' click on 'Default' and hit 'Apply'. After you reset to default mode you'll be able to delete the other theme that you had and apply dark mode.
I was able to choose dark mode following the steps you laid out. However, the home screen did not turn dark. The lock screen and messages did, but not the home screen.