Your application has a dark background is the hottest trend right now, and all application developers who chimed in. Google has released a dark theme for many applications, and now Gmail will get it too. It must be hit with Android 10 users and iOS 11 or higher, but it may take couple of weeks before it gets to all devices.
If you use a smartphone Pixel Google, Gmail automatically in dark theme after you have activated Battery Saver. It also will honor system default settings, if you decide to have all black everything. However, if you want ONLY the Gmail app to be dark, simply go to Settings -> Themes -> Dark.
Apple iOS users 11 or 12 also should look for options in the Settings, while those who are already rocking the latest version, will receive automatically if they have prepared a total blackout for UI. Again – if you want to just be in the dark Gmail Mode, looking for the switch in Settings.
Dark mode will be available both for regular customers and G Suite account after you update your application.