Google is currently rolling out this update and it may take a while to get to some devices and regions. If your device has the feature, you can manually turn on the dark mode by following these steps:
Once the feature is added to your device, it is automatically set to system default which means that your phone’s dark mode setting will control the dark mode in the app. You can also preview a document in light mode, as Google has added an option in the app to help you toggle between light and dark modes with your files without altering the app settings. The company is yet to make any mention of iOS devices getting this update. References
