Just open Settings > go to Display and Brightness > and Tap Dark to switch to Dark mode. If you're running an older version of iOS, go to Settings in the Gmail app > then Theme > and select “Dark”. Note: If your device is already running iOS 13 or later, you can set dark theme as the system default on your device, and then all apps that adopt the default system will automatically go dark. Gmail will, by default, automatically respond to this system default setting. Just go to Settings > scroll down and tap on the Display option > and tap on the Dark theme toggle.
Note: If your mobile device is running Android 10 or later, you can set dark theme as the system default on your device, and then all supported apps that adopt the default system will automatically go dark. Tap Dark (or, if you've already set the dark theme as your system default, tap System default).
However, you can also turn it on separately in the Gmail mobile app. If you already use Android 10/11+ or iOS 13/14+ and have dark mode turned on at the system level, the Gmail app will auto-adjust to dark mode. How to enable dark mode on Gmail's mobile app
