Hi Sebastian Hoppe,
which background do you mean?
On the top, you can see the status bar, which on Android can be set in the platform-specific code, e.g. MainActivity.cs via this method:
Window?.SetStatusBarColor(Colors.Orange.ToAndroid());
On iOS, this shouldn't be necessary, usually the page background color is used, which can be set in a platform-independent fashion like this:
<ContentPage
BackgroundColor="Orange">
</ContentPage>
I hope this helps.
I'm thinking about writing a separate blog post about that.
Cheers
Julian