Usually, when Google introduces a new feature, it takes a few weeks for it to reach the masses. While both Android 10 and iOS 13 introduced system-wide dark mode, developers still have to update apps to be compatible. That was the case with Gmail, which wasn’t dark mode compatible when Android 10 first launched.
If you’ve enabled Dark Theme on Android 10, the Gmail app should sport dark mode automatically. But if you want to set it manually, you can just go to Settings > Theme. As for iOS and ipadOS, it appears the feature is still gradually rolling out. (In its announcement blog, Google noted that while extended rollout began on September 24, it could take over 15 days for it to be available for everyone.)
That said, even though Gmail’s dark theme is now widely available on Android 10 devices, the majority of Android handsets are still running older versions of Android. Though, if you were thinking of getting the Pixel 4, then yes — you’d have Android 10 out of the box (as do the other Pixel phones).
Purportedly, it helps extend battery life on phones with OLED screens and anecdotally, people say it helps reduce eye strain and migraines. Whether that’s actually true is up for debate. In the meantime, however, the latest version of Opera for Android has decided dark mode is so last year. Instead, Opera has decided it’s going to offer colour variants for both light and dark versions of its mobile browser.