If you're using Shell, you can navigate to the Styles.xaml file and then find the style setter for the Shell.BackgroundColor property:
<Setter Property="Shell.BackgroundColor" Value="{AppThemeBinding Light=#123456, Dark=#654321" />
Or when you use a NavigationPage or TabbedPage, you can set the BarBackgroundColor:
<Setter Property="BarBackgroundColor" Value="{AppThemeBinding Light={StaticResource Primary}, Dark={StaticResource Gray950}}" />
Of course, you don't need to use the AppThemeBinding, you can also set it to a single, static value.