Setting Dark Mode in the System Preferences will not enable Dark Mode as it does other Apps: this preference must be set in gMail itself.
Unless the "Theme" menu is present in gMail's settings, you CANNOT set gMail to use Dark Mode. You are wasting your time- I know, I've wasted half a day screwing around with this flaky nonsense.
Indeed, Googling produced links where I read about numerous other equally frustrated folks pulling their hair out of their heads trying to get Dark Mode to work.
The Dark Mode preference must be set locally in gMail and that requires the "Theme" menu. If it's not there, you're out of luck. I've posted this to show other folks where I've been to save them wasting half a day of their lives. HTH-