Android 9.0 will include a built-in dark mode theme that users will be able to enable from the settings menu. While there is not much information about the dark mode, it was recently mentioned in the Google Issue Tracker that the feature would be added in a future Android release. While "future Android release" doesn't explicitly mean that it will be Android 9.0 that gets this feature, it is worth noting that the Issue Tracker thread that this response was posted in was about Android P, otherwise known as Android 9.0, and was labeled Dark Mode or Night Mode in Android 9.0, with the first post coming from a user who requests that a built-in Dark Mode be added into Android 9.0 once it releases.
This alone should be enough information to at least spark the interest in this feature coming to the system as a native feature users will have access to without having to install any third-party applications from the Play Store, though it should also be noted that a response from a Google employee does not mean that it will be added, and won't otherwise be confirmed until Google states that this will be a feature to be expected in the upcoming Android version.
According to the responses on the Issue Tracker, adding a native Dark Mode into the Android system settings was being looked into by engineers working on the software, with the same person who mentioned that engineers were made aware of the request having stated that the feature would be added in, so while this is not entirely concrete just yet it is as close as it's going to get for the time being until Google officially announces Android 9.0, which could happen at Google I/O this year. That said there is no guarantee that it would announce this particular feature at the event, as Google tends to only open up about some of the new features and changes in the beginning. In addition to a native Dark Mode, it's also been reported that Android 9.0 would include some locked APIs, and that the software would attempt to block apps that were sitting idle and not being used in an effort to increase privacy and security of the OS.
According to a newer post in the same Issue Tracker thread, Google is actually not planning on implementing a native Dark Mode for Android in the system settings that will be user-facing that could be toggled on or off. What it has done is add in a night mode UI qualifier through the developer options that is meant for developers to help them test adding night mode to their own apps.