If your app uses any kind of sign-in method then you can customize the way your sign-in screen looks by uploading your own background, logo, greeting & description.
Your sign in screen will default to what you see below + whatever accent color you have chosen for your app. To start customizing your sign-in screen visit Settings/Sign-in. Here you can customize the background, add a logo, change the 'Welcome' text and also the description (the text below 'Welcome').
You can change the greeting and the description in the Text section. After you've added the your custom text, you can change between a black or a white theme.
Your choice for this is likely to be affected by your background. If your background is quite dark, you may want to use the White Theme and if your background is quite light, you may want to use the Dark Theme.
In the branding section you can upload a logo and a background. The background image will replace the background accent color of your app and your logo will replace the title at the top of this screen.
After you have uploaded these you can experiment with the Gradient Overlay settings and Black & White Themes to make sure your text works well with your image.
You can also enable the setting to use your background image in your app's loading screen.