Phonebook/Services/ThemeService.cs

using System.Reflection;

namespace Phonebook.Services;

public enum AppThemeType { Green, Blue, Dark, Light }

public class ThemeService
{
    private readonly Application _app;
    private readonly string _prefsKey = "user_theme_preference";
    
    private static readonly Dictionary<AppThemeType, string> ThemePaths = new()
    {
        { AppThemeType.Green, "Themes/GreenTheme.xaml" },
        { AppThemeType.Blue, "Themes/BlueTheme.xaml" },
        { AppThemeType.Dark, "Themes/DarkTheme.xaml" },
        { AppThemeType.Light, "Themes/GreenTheme.xaml" }
    };

    public ThemeService()
    {
        _app = Application.Current ?? throw new NullReferenceException();
    }

    public AppThemeType CurrentTheme { get; private set; } = AppThemeType.Green;

    public void LoadSavedTheme()
    {
        var saved = Preferences.Get(_prefsKey, string.Empty);
        if (Enum.TryParse<AppThemeType>(saved, out var theme))
        {
            ApplyTheme(theme);
        }
    }

    public async Task ApplyTheme(AppThemeType themeType)
    {
        if (!ThemePaths.TryGetValue(themeType, out var resourcePath))
            return;
        
        // Remove existing theme dictionary
        var existing = _app.Resources.MergedDictionaries
            .FirstOrDefault(d => d.Source?.OriginalString.Contains("Themes/") == true);
        if (existing != null)
            _app.Resources.MergedDictionaries.Remove(existing);
        
        var newTheme = new ResourceDictionary();

        await using var stream = await FileSystem.Current.OpenAppPackageFileAsync(resourcePath);
        using var streamReader = new StreamReader(stream);
        newTheme.LoadFromXaml(streamReader.ReadToEnd());
        
        _app.Resources.MergedDictionaries.Add(newTheme);

        CurrentTheme = themeType;
        Preferences.Set(_prefsKey, themeType.ToString());
        
        // Notify UI if needed
        ThemeChanged?.Invoke(this, EventArgs.Empty);
    }

    public IEnumerable<AppThemeType> AvailableThemes => 
        Enum.GetValues<AppThemeType>();

    public event EventHandler? ThemeChanged;
}