Can Notification Center be made black/dark in macOS Sierra beta?
Solution 1:
Since the macOS Sierra update (Including Betas) the Notification Panel no longer uses the Dark Mode option, therefore it is permanently in light mode. This is an intended feature.
When Dark Mode is enabled on macOS Mojave or later, it will appear dark:
Solution 2:
Apple haven't made the notification center darkmodus yet after launching macOS Sierra. This withholds some users to update.