If you updated to Android 10 and were surprised to not see the dark theme in Gmail yet, it might be worth taking a second look. Some users have reported a new “themes” option in the Gmail app settings that includes a dark mode setting. The linked article below will show how to check if Gmail’s new dark mode is available for you.
If you still don't have it you can do the following with a rooted phone which work3ed for me:
1. Go to data\data\com.google.android.gm\shared_prefs\FlagPrefs.xml
2. Open it with an editor then search for "DarkThemeSupport" (should be one of the first flags) and change to True. Save.
3. Force close Gmail and reopen. Theme support should be now there in the General Settings.
Despite teasing expanded dark mode support ahead of Android 10's release, many users who installed the update noted that several Google apps still lack dark themes, including its Gmail app. However, it looks as if those missing dark theme options are finally starting to appear.