Each app’s dark theme can be forced to dark or light or follow your system settings. To change that setting, head into the app’s Menu>Settings>Theme and then choose the option that fits your needs. If you want it to auto-switch as your phone’s system does, choose “System default.”
You should also be able to quickly toggle themes when viewing an individual document/sheet/slide by tapping the 3-dot menu (top right) and then looking for “View in light/dark theme.”
The rollout for dark theme of each app should be rolling out right now.