UPDATE:
Setting Dark Mode in the System Preferences willWas surprised to see Dark Mode magically working on my iPhone's gMail. I had not enable Dark Mode asfiddled with anything lately, but voila, there it does other Apps: this preference must be set in gMail itselfwas.
Unless the "Theme" menu is present in gMail's settings However, youwhen I checked my iPads, these did CANNOTNOT set gMail to use have Dark Mode enabled. Checked the versions on the iPads and these had the same one as the iPhone: v.6.0.200412. So what was different? Dark ModeI rebooted the iPhone recently. You are wasting your time- I know, I've wasted half a day screwing around with this flaky nonsensebut NOT my iPads.
Indeed, Googling produced links where I read about numerous other equally frustrated folks pulling their hair outUpon reboot of their heads trying to get Dark Mode to workthe iPads, Dark Mode was now working on gMail on all my devices.
How to get Dark Mode working: (3) Steps
- Set Dark Mode as the system-wide preference in: "System Preferences" > "Display & Brightness" > "Appearance" > "Dark"
- As @Lulucmy rightly noted in his answer, Google must enable the feature on their side or Dark Mode ain't happening for you.
- Reboot the device periodically: Google will not notify you they are enabling Dark Mode. I had periodically closed gMail from time to time since I last updated it 01 May, and Dark Mode was still not working in gMail. Only when I rebooted my iPhone did the change seem to be effective.
TheSo Dark Mode Dark Mode preference must be set locally in gMail and that requires the "Themewill" menu. If it's not there respect the global System Preference, you're out of luck. I've posted this to show other folksbut only where I've beenGoogle has enabled it on their side. So I guess patience is also required for those eager to save them wasting half a day ofextend their livesbattery life using gMail in Dark Mode. HTH
Anyhoo, my observations on how my own Dark Mode Hell was resolved. Hope this saves others cycles trying to resolve the issue-