In order to activate dark mode for the Google Discover feed, it is usually necessary to switch on the dark theme for either the entire mobile device or the Google app since Discover generally adopts the system setting of the device. However, some phones like Pixels have this feature directly integrated and you just need to swipe left. To find out if your phone has a universal Dark Theme toggle, check the Display settings on your phone or go to the Google app, tap your profile picture, then go to Settings, then General, and finally, Theme to set it there if available, making sure that you are signed in.