Theme setting missing in iPhone Gmail app
Indeed there doesn’t seem to be a Gmail setting to turn off syncing to the iOS theme (I also looked within the settings of specific mail accounts). This is something Google would need to implement - you can provide this feedback to them from the Gmail app (“Send feedback” is below “Settings” in the side navigation).
As a workaround, you can add “Dark Mode” to your iOS Control Center in System Preferences. This will allow toggling the global theme quickly.
I also noticed that the Outlook iOS app supports overriding the system theme, if you’re looking for an alternative to Gmail.