How to update StatusBar color based on dark/light theme

The UISettings.ColorValuesChanged event can be used to listen for notifications when dark mode is enabled or disabled at the system level.

Example

The complete sample code can be found in the StatusBarThemeColor sample in Uno.Samples GitHub repository.

Step-by-step instructions

Create a new Uno Platform application, following the instructions in Get Started guide.
In MainPage.xaml, add a <CommandBar>:

On iOS, the status bar color cannot be set directly, so it is done via a CommandBar placed in the page. You could also use any XAML element like <Grid> or <Border> to achieve a similar effect, if your application doesn't use navigation or doesn't use native navigation. This is because the page content can go under the status bar. In fact, you usually have to add padding to avoid that (see next step).
```
<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto" />
        <RowDefinition Height="*" />
    </Grid.RowDefinitions>

    <CommandBar Grid.Row="0"
                x:Name="MyCommandBar"
                Content="MainPage" />

    <StackPanel Grid.Row="1">
        <TextBlock Text="Hello, world!"
                    Margin="20"
                    FontSize="30" />
    </StackPanel>
</Grid>
```
Add a VisibleBoundsPadding.PaddingMask to the root <Grid> to prevent the content being partially covered by the status and command bars:
```
<Page ...
      xmlns:toolkit="using:Uno.UI.Toolkit"
      ...>

    <Grid toolkit:VisibleBoundsPadding.PaddingMask="Top">
```

In MainPage.xaml.cs, expose the MyCommandBar which will be referenced in a later step:

public static MainPage Instance { get; private set; }

public MainPage()
{
    Instance = this;

    this.InitializeComponent();
}

public CommandBar GetCommandBar() => MyCommandBar;

In App.cs, enable the native frame navigation to use the native style for CommandBar, instead of the UWP style. Make sure that the #if ... #endif block is before this.InitializeComponent():

public App()
{
    /***/

#if __IOS__ || __ANDROID__
    // Enable native frame navigation.
    Uno.UI.FeatureConfiguration.Style.ConfigureNativeFrameNavigation();
#endif

    this.InitializeComponent();
    this.Suspending += OnSuspending;
}

Subscribe to the UISettings.ColorValuesChanged event from App.cs:

Note that the instance of UISettings is kept to prevent it from being disposed when going out of scope, which would otherwise dispose the event subscription.

public sealed partial class App : Application
{
    private UISettings _uiSettings;
    private bool? _wasDarkMode = null;

    /* ... */

    protected override void OnLaunched(LaunchActivatedEventArgs e)
    {
        /* existing code... */

        // add this line at the end of OnLaunched:
        ConfigureStatusBar();
    }

    /* ... */

    private void ConfigureStatusBar()
    {
        // Listen for the system theme changes.
        _uiSettings = new UISettings();
        _uiSettings.ColorValuesChanged += (s, e) =>
        {
#if __ANDROID__
            var backgroundColor = _uiSettings.GetColorValue(UIColorType.Background);
            var isDarkMode = backgroundColor == Windows.UI.Colors.Black;

            // Prevent deadlock as setting StatusBar.ForegroundColor will also trigger this event.
            if (_wasDarkMode == isDarkMode) return;
            _wasDarkMode = isDarkMode;
#endif

            UpdateStatusBar();
        };

#if __IOS__
        // Force an update when the app is launched.
        UpdateStatusBar();
#endif
    }
}

Implement the UpdateStatusBar() method:

private void UpdateStatusBar()
{
    // === 1. Determine the current theme from the background value,
    // which is calculated from the theme and can only be black or white.
    var backgroundColor = _uiSettings.GetColorValue(UIColorType.Background);
    var isDarkMode = backgroundColor == Windows.UI.Colors.Black;

#if __IOS__ || __ANDROID__
    // === 2. Set the foreground color.
    // note: The foreground color can only be set to a "dark/light" value. See uno remarks on StatusBar.ForegroundColor.
    // note: For ios in dark mode, setting this value will have no effect.
    var foreground = isDarkMode ? Windows.UI.Colors.White : Windows.UI.Colors.Black;
    Windows.UI.ViewManagement.StatusBar.GetForCurrentView().ForegroundColor = foreground;

    // === 3. Set the background color.
    var background = isDarkMode ? Windows.UI.Colors.MidnightBlue : Windows.UI.Colors.SkyBlue;
#if __ANDROID__
    // On Android, this is done by calling Window.SetStatusBarColor.
    if (Uno.UI.ContextHelper.Current is Android.App.Activity activity)
    {
        activity.Window.SetStatusBarColor(background);
    }
#endif
    // On iOS, this is done via the native CommandBar which goes under the status bar.
    // For android, we will also update the CommandBar just for consistency.
    if (MainPage.Instance.GetCommandBar() is CommandBar commandBar)
    {
        commandBar.Foreground = new SolidColorBrush(foreground); // controls the color for the "MainPage" page title
        commandBar.Background = new SolidColorBrush(background);
    }
#endif
}

Change the theme of your device to validate the result.

Concluding remarks

This is just one way to show how it could be done. Alternatively, depending on the application, the following options can also be considered:

Attached Property on the CommandBar (when multiple pages are used)
Xaml Grid/Border/Image that goes under the StatusBar (with WindowManagerFlags.TranslucentStatus on Android, no additional change on iOS)