Currently, Gmail’s dark mode is available only on Android 10 devices. Make sure you have an updated Gmail app. To enable Dark Theme in the Gmail app, tap on the hamburger menu at the top left and tap on Settings> General settings> Theme>Dark. This will enable a dark theme on Gmail. I personally love Dark theme, not only it saves battery life on OLED screens but also is less strain on eyes. So enable it today if you are on Latest Android or iOS version.