Apple recently switched from the default for most apps being provided with a white background, to allowing the user to select a 'Light' theme or a 'Dark' theme. You can change this by going to Settings -> Display & Brightness -> Appearance - Options for Light and Dark. If you are in Dark mode you will see only a black screen on start up.
For the Bluetooth to work you must allow your phone to use Bluetooth and Location Settings. If you do not allow this your phone will not pair with your motor. For all Android phones go to Settings, then Apps, Then Lift Tech. For all Iphone's go to Settings then Lift Tech.