On Tuesday (19.05.20) the company announced that it had begun the process of rolling out the new feature and it expects all Android and iOS users to have the option by the end of the week.
A message on Google's Search Liaison Twitter account stated: "Starting today & fully rolling out this week, dark mode is now available for the Google App on Android 10 and iOS 12/13. By default, the Google App will reflect your device's system settings in Android 10 & iOS 13. If that's set to a dark theme, the app will be in dark mode..."
Until it is fully rolled out, users can manually change from dark to light mode in settings.
Google explained: "You can also choose to have the Google App use a light or dark mode different from your system default. To do this, go into the Google App settings and choose the mode that you prefer. This is also how to set dark mode for the Google App in iOS 12 (sic)."