Google is in the process to start rolling out the dark mode of Gmail over Android. The changes in the app are available in the latest version of the Gmail app(2019.08.18.267044774) available over the Google play store.
Settings
For Pixel Users with Android 10 need to download the current version of the Gmail app in order to take advantage of the dark mode. Then Browse Settings> Theme>Light/Dark mode/System Default.
What Changes does it make?
The dark mode will turn the background Dark Grey of Gmail app along with inbox & all the emails.
Note: Google first started testing this dark theme over Android earlier this year.