Google has started the roll out dark mode for Gmail starting on both Android and iOS devices. If you are using Android 10 or iOS 13 and have dark mode switched on at the system level, Gmail will automatically adjust to dark mode.
Also, if you are using a Pixel phone, the dark mode will be activated once you set your phone to battery saver mode. However, if you are using an earlier version of Android or iOS, the dark mode will still be available to you but you’ll have to change a few things in your settings.
One can check whether they have the dark mode available to them by opening Gmail, click on settings where you will see a new option named “Theme” where you can be able to switch on the dark mode. However, if you are not seeing the option, fret not, as there is an extended roll out of the feature so it will reach you eventually.