If you’re already been updated to Android 10 (whether or not you’re having issues) you might be one of the first in line for dark mode for Google’s email app. It’s not rolling out for everyone yet, but many users are reporting seeing a toggle for dark mode on Gmail.
It’s pretty strange to only be available to Android 10, even without the full settings. With Android 10 you have the option to change between light and dark, plus an automatic mode that follows your system-wide themes. That’s a nifty trick and really useful, but users on older phones and software should at least be able to toggle between light and dark mode manually, even if they can’t use the automatic changes.
To be fair, there is a potential workaround if you’re on older versions of Android. If you’re rooted, you can side load the newer Gmail APK and then change some flags in some files for Gmail, which could trigger the dark mode toggles, depending on your phone. That might be more headache than it’s worth, though, but you’ve got options.
source: XDA Developers