When the developers were discussing the launch of Android 10, they highlighted the fact that the Android 10 will have a new system-wide dark theme. The built-in dark mode toggle was supposed to make the Settings, SystemUI, and framework accentuated with a darker shade of Material Design colors.
Following this, Google had announced that it would apply the dark theme to all of its apps but Google Play Store and Gmail app had not received the dark modes before the release of Android 10. Notably, the Gmail dark mode is finally available for both the Google Play Store and the Gmail app.
Now, in the Gmail app for Android, there is an option for changing the theme in general settings, and the users will have the option to choose from a light, dark, and system default option. If the Gmail dark mode is enabled, all the background of the Gmail app including the background of inbox ends up donning a dark gray theme.
Microsoft was the first one who brought out the Dark theme for its mail app, but Google did not lag too far behind. The version which will support and have the option of enabling the dark mode is available on the Google Play Store.
It can also be downloaded from APKMirror. If anyone is interested in enabling the dark mode manually, they would need the root access to set the DarkThemeSupport__dark_theme_support boolean flag to true in /data/data/com.google.android.gm/shared_prefs/FlagPrefs.xml. After the flag is set to true, the file needs to be saved, and the user needs to force close the Gmail app.
However, it is not guaranteed that by following this method, one can enable the dark mode in other OS systems other than Android 10. One week after the launch of Android 10, Google kept its promise and its Play Store, and the Gmail app will finally get the characteristic update of Android 10.