maui-theming（毛伊岛主题）

.NET MAUI Theming

Apply light/dark mode support, custom branded themes, and runtime theme switching in.NET MAUI apps using AppThemeBinding, ResourceDictionary swapping, and system theme detection APIs.

When to Use

• Adding light and dark mode support to a.NET MAUI app
• Creating custom branded themes with ResourceDictionary
• Detecting and responding to system theme changes at runtime
• Letting users choose a preferred theme (light, dark, or system default)
• Combining OS-driven theme response with custom color palettes

When Not to Use

• Localization or language switching — see .NET MAUI localization docs
• Accessibility-specific visual adjustments — see .NET MAUI accessibility docs
• App icon or splash screen configuration — see .NET MAUI app icon docs
• Bootstrap-style class theming — see the Plugin.Maui.BootstrapTheme NuGet package

Inputs

• A.NET MAUI project targeting.NET 8 or later
• XAML pages or C# UI code that need theme-aware styling

Workflow

1. Detect the current theme approach in the project (AppThemeBinding, ResourceDictionary, or none).
2. Choose the appropriate strategy: AppThemeBinding for simple light/dark, ResourceDictionary swap for custom/multiple themes, or both combined.
3. Define theme resources — inline AppThemeBinding values or separate ResourceDictionary files with matching keys.
4. Replace hardcoded colors with DynamicResource bindings (or AppThemeBinding markup) throughout XAML pages.
5. Add system theme detection via Application.Current.RequestedTheme and the RequestedThemeChanged event.
6. Implement user preference persistence with Preferences.Set / Preferences.Get and apply on startup.
7. Verify Android ConfigChanges.UiMode is set on MainActivity to avoid activity restarts on theme change.
8. Test both light and dark themes on at least one target platform, confirming all UI elements respond correctly.

Choosing an Approach

AppThemeBinding (OS Light/Dark)

AppThemeBinding selects a value based on the current system theme. It supports Light, Dark, and an optional Default fallback.

XAML

<Label Text="Themed text"
       TextColor="{AppThemeBinding Light=Green, Dark=Red}"
       BackgroundColor="{AppThemeBinding Light=White, Dark=Black}" />

<!-- With resource references -->
<Label TextColor="{AppThemeBinding Light={StaticResource LightPrimary},
                                   Dark={StaticResource DarkPrimary}}" />

C# Extension Methods

var label = new Label();

// Color-specific helper
label.SetAppThemeColor(Label.TextColorProperty, Colors.Green, Colors.Red);

// Generic helper for any bindable property type
label.SetAppTheme<Color>(Label.TextColorProperty, Colors.Green, Colors.Red);

ResourceDictionary Theming (Custom Themes)

Use separate ResourceDictionary files with matching keys to define themes, then swap them at runtime.

Step 1 — Define Theme Dictionaries

When using compiled XAML with x:Class (as shown below), each dictionary needs a code-behind that calls InitializeComponent(). Dictionaries loaded via Source without x:Class do not need code-behind.

LightTheme.xaml

<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>

LightTheme.xaml.cs

namespace MyApp.Themes;

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

Create a matching DarkTheme.xaml / DarkTheme.xaml.cs with the same keys and different values.

Step 2 — Consume with DynamicResource

Use DynamicResource so values update when the dictionary is swapped at runtime:

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

Step 3 — Switch Themes at Runtime

void ApplyTheme(ResourceDictionary theme)
{
    // Assumes theme dictionaries are the only merged dictionaries.
    // If your App.xaml merges non-theme dictionaries (e.g., converters),
    // move them to Application.Resources directly instead.
    var mergedDictionaries = Application.Current!.Resources.MergedDictionaries;
    mergedDictionaries.Clear();
    mergedDictionaries.Add(theme);
}

// Usage
ApplyTheme(new DarkTheme());

System Theme Detection

Read the Current Theme

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

Override the System Theme

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

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

React to Theme Changes

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

Combining Both Approaches

Use AppThemeBinding with DynamicResource values for maximum flexibility:

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

Or react to system changes and swap full dictionaries:

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

Saving and Restoring User Preference

Store the user's choice with Preferences and apply it on startup:

// 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
};

Common Pitfalls

Android: ConfigChanges.UiMode is Required

MainActivity must include ConfigChanges.UiMode or theme-change events will not fire and the activity restarts instead of handling the change gracefully:

[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 { }

Without UiMode, toggling dark mode in Android settings causes a full activity restart — losing navigation state and appearing as a crash.

DynamicResource vs StaticResource

When using ResourceDictionary theme switching, you must use DynamicResource:

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

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

Hardcoded Colors Break Theming

Avoid inline color values on elements that should respect the theme:

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

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

CSS Themes Cannot Be Swapped at Runtime

.NET MAUI supports CSS styling, but CSS-based themes cannot be swapped dynamically. Use ResourceDictionary theming for runtime switching.

Theme Keys Must Match Across Dictionaries

Every x:Key used in one theme dictionary must exist in all other theme dictionaries. A missing key causes a silent fallback to the default value, leading to inconsistent appearance.

Platform Support

Quick Reference

• OS light/dark → AppThemeBinding markup extension
• Theme colors in C# → SetAppThemeColor(), SetAppTheme<T>()
• Read OS theme → Application.Current.RequestedTheme
• Force theme → Application.Current.UserAppTheme = AppTheme.Dark
• Theme changes → RequestedThemeChanged event
• Custom switching → Swap ResourceDictionary in MergedDictionaries
• Runtime bindings → DynamicResource (not StaticResource)
• Persist choice → Preferences.Set / Preferences.Get