Signs of this happening have been visible in the phone's settings a while back. Not long ago, a dark theme Gmail widget also became available, to everyone. All you had to do is set your general system theme to dark mode, and you were able to utilize a dark Gmail widget.
Only available on Android 10, for now
Dark Mode for Gmail did not arrive in full, though, at least not until now. Still, this update is only available to Android 10 users, it seems. Those of you who are running earlier versions of Android will have to wait a bit longer, unfortunately.
It is worth noting that this update is a bit late to the party. Google promised to update all of its major apps with the dark theme option before Android 10 arrives. Dark mode for Gmail is not the only exception, as the dark mode did not arrive to the Play Store either. Still, Google did a good job when it comes to updating its apps to the dark mode, as many of them received the update prior to Android 10 release.
Dark mode is available for Chrome, YouTube, Contacts, Phone, Snapseed, Google Keep (aka Keep Notes), and many more applications. Gmail is getting it now, and Google Play Store will also get it in the near future.
Having said that, Gmail is getting dark mode with version 2019.08.18.267044774, at least that's what was noted on Reddit. If you check the gallery down below, you can see what this dark theme looks like.
Google added an option to the "General Settings" menu in Gmail's Settings. You can choose whether you'd like your Gmail theme to be Light, Dark, or System Default. If you choose the latter, Gmail will pick your general system theme.
Dark gray background with white / light gray letters
Once you enable Dark Theme in Gmail, the app's background will become dark gray. This is actually a similar approach Google took in all of its other apps, so the background will not be pitch black.
The text will be white, in some cases light gray, while some blue accents are also being used by the company.
Now, there is a possibility that this option may not appear to you even though you have Android 10. If you want to enable it manually, that is possible as well, but you'll need to have root access.
If you have root access, you can set the DarkThemeSupport__dark_theme_support boolean flag to true in /data/data/com.google.android.gm/shared_prefs/FlagPrefs.xml. Once you do that, save the file, and then force close the Gmail app.
This approach will work for devices who run Android 10, but don't seem to have this option enabled. Some Essential PH-1 users reported that the option was not available for them, so they were able to enable it this way. Do note that this approach probably won't work if you're not running Android 10, though.