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.
Default sign-in screen
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 Screen. Here you can customize the background, add a logo, change the 'Welcome' text, and also the description (the text below 'Welcome').
We can change the greeting and the description in the Text section. After we've added our custom text, we can change between a black or a white theme.
Our choice for this is likely to be affected by our background. If our background is quite dark, you may want to use the White Theme and if our background is quite light, we may want to use the Dark Theme.
In the branding section, we can upload a logo and a background. The background image will replace the background accent color of our app and our logo will replace the title at the top of this screen.
After we have uploaded these we can experiment with the Gradient Overlay settings and Black & White Themes to make sure our text works well with our image.
We can also enable the setting to use your background image in your app's loading screen.