According to a post in the Chromium bug tracker made by Product Manager at Google, Lukasz Zbylut, Android Q may be getting a universal dark mode. In the post, he states that the feature is approved for Android Q and that "The Q team wants to ensure that all preloaded apps support dark mode natively. In order to ship dark mode successfully, we need all UI elements to be ideally themed dark by May 2019."
Google has been adding a dark mode to many of its preloaded apps on Android including YouTube and may even support the Windows 10 dark mode in an upcoming build of Chrome. Whilst OEMs such as Samsung and OnePlus allow for third-party overlays (themes) on their phones, Google so far has been less welcoming of the feature. However, if the feature does finally come to Android Q, it would be a nice addition which would be welcomed by many (including me).
Since the post was spotted, Google has changed the permissions for it so that non-Googlers will no longer be able to access it which has led Android Police to believe that this confirms the upcoming option of a dark mode in Android Q. From the post it does not seem like Google will be enabling third-party overlays a la Samsung, instead giving you a choice between the current bright white theme and a dark theme.