In the newest Android 10 and iOS 13 update, both OS from different manufacturers adds in dark mode as official settings available from the mobile OS. And since then Gmail has been introducing Dark Mode which will be released in the future update.
According to a report by 9to5Google, the DarkMode changes the background colour to black with text color remained at grey. All settings and contact name will be changed to white. Once the email is read, the text will change to white too!
According to G Suite updates, the Dark Mode is now available in Android for iOS for testing. For those who are running Android 10, once the dark mode is activation, the Gmail will opt for dark mode for its display. Similar for iOS 13 users.