The option for the dark theme can be found under Settings -> Dark theme. You can set it to always or never be on, as well as have it automatically turn on with the battery saver and/or at night. The latter doesn't appear to be linked to the Night Light or Grayscale settings on Android 9 Pie, neither is it dependent on ambient light, so it might be that Google is using your location to judge when night time is for you and changing the theme accordingly.
While Google News used to show dark backgrounds on some articles, this new switch turns the entire interface around. It even flips the status bar and navigation bar to black. I'll let you feast your eyes on all the darkness.
There don't appear to be any other major changes in News 5.5 — only some reorganization of the settings. The update is slowly rolling out on the Play Store, so if you want to get it now, you can grab it from APK Mirror.