Google is rolling out a dark theme to Gmail for Android and iOS users. The feature is currently rolling out to all Gmail users today, September 24th. Users will be able to manually enable the dark theme by going to Settings -> Themes and then select Dark. Users of iOS 13 and Android who have the Dark theme enabled on the phone will also see it automatically enabled in the Gmail app. When Battery Saver mode is enabled on a Pixel, Gmail will automatically default to the Dark.
Source: https://www.bleepingcomputer.com/news/google/gmail-for-android-and-ios-gets-a-dark-theme/