Maui Theming

<ResourceDictionary xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
                    x:Class="MyApp.Themes.LightTheme">
    <Color x:Key="PageBackgroundColor">White</Color>
    <Color x:Key="PrimaryTextColor">#333333</Color>
    <Color x:Key="AccentColor">#2196F3</Color>
</ResourceDictionary>

namespace MyApp.Themes;

public partial class LightTheme : ResourceDictionary
{
    public LightTheme() => InitializeComponent();
}

<ContentPage BackgroundColor="{DynamicResource PageBackgroundColor}">
    <Label Text="Hello"
           TextColor="{DynamicResource PrimaryTextColor}" />
    <Button Text="Action"
            BackgroundColor="{DynamicResource AccentColor}" />
</ContentPage>

static ResourceDictionary? _currentTheme;

void ApplyTheme(ResourceDictionary theme)
{
    var merged = Application.Current!.Resources.MergedDictionaries;

    // ✅ Remove ONLY the previous theme — Colors.xaml / Styles.xaml survive
    if (_currentTheme is not null)
        merged.Remove(_currentTheme);

    merged.Add(theme);
    _currentTheme = theme;
}

// Usage
ApplyTheme(new DarkTheme());

// ❌ Destroys the app's Colors.xaml and Styles.xaml along with the old theme
var merged = Application.Current!.Resources.MergedDictionaries;
merged.Clear();
merged.Add(theme);

AppTheme currentTheme = Application.Current!.RequestedTheme;
// Returns AppTheme.Light, AppTheme.Dark, or AppTheme.Unspecified

// Force dark mode regardless of OS setting
Application.Current!.UserAppTheme = AppTheme.Dark;

// Reset to follow system theme
Application.Current!.UserAppTheme = AppTheme.Unspecified;

Application.Current!.RequestedThemeChanged += (s, e) =>
{
    AppTheme newTheme = e.RequestedTheme;
    // Update UI or switch ResourceDictionaries
};

<Label TextColor="{AppThemeBinding
    Light={DynamicResource LightPrimary},
    Dark={DynamicResource DarkPrimary}}" />

Application.Current!.RequestedThemeChanged += (s, e) =>
{
    ApplyTheme(e.RequestedTheme == AppTheme.Dark
        ? new DarkTheme()
        : new LightTheme());
};

// Save choice
Preferences.Set("AppTheme", "Dark");

// Restore on startup (in App constructor or CreateWindow)
var saved = Preferences.Get("AppTheme", "System");
Application.Current!.UserAppTheme = saved switch
{
    "Light" => AppTheme.Light,
    "Dark"  => AppTheme.Dark,
    _       => AppTheme.Unspecified
};

[Activity(Theme = "@style/Maui.SplashTheme",
          MainLauncher = true,
          ConfigurationChanges = ConfigChanges.ScreenSize
                               | ConfigChanges.Orientation
                               | ConfigChanges.UiMode  // ← Required for theme detection
                               | ConfigChanges.ScreenLayout
                               | ConfigChanges.SmallestScreenSize
                               | ConfigChanges.Density)]
public class MainActivity : MauiAppCompatActivity { }

<!-- ✅ Updates when theme dictionary changes -->
<Label TextColor="{DynamicResource PrimaryTextColor}" />

<!-- ❌ Frozen at first load — won't update on theme switch -->
<Label TextColor="{StaticResource PrimaryTextColor}" />

static ResourceDictionary? _currentTheme;

void ApplyTheme(bool useDark)
{
    var merged = Application.Current!.Resources.MergedDictionaries;

    // Remove only the previous theme — never Clear(), which also wipes
    // the template's Colors.xaml / Styles.xaml
    if (_currentTheme is not null)
        merged.Remove(_currentTheme);

    _currentTheme = useDark ? new DarkTheme() : new LightTheme();
    merged.Add(_currentTheme);
}

// React to the OS switching light/dark
Application.Current!.RequestedThemeChanged += (s, e) =>
    ApplyTheme(e.RequestedTheme == AppTheme.Dark);

<!-- ❌ Will not change with theme -->
<Label TextColor="#333333" />

<!-- ✅ Theme-aware -->
<Label TextColor="{DynamicResource PrimaryTextColor}" />