marechai/Marechai.App/Services/ColorThemeService.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Windows.Storage;
using Microsoft.UI.Xaml;
using Uno.Extensions.Toolkit;

namespace Marechai.App.Services;

public interface IColorThemeService
{
    string                CurrentColorTheme    { get; }
    IReadOnlyList<string> AvailableColorThemes { get; }
    void                  ApplyColorTheme(string        themeName);
    void                  SetThemeService(IThemeService themeService);
    void                  ReapplyCurrentTheme();
}

public class ColorThemeService : IColorThemeService
{
    private const string        COLOR_THEME_KEY = "ColorTheme";
    private const string        DEFAULT_THEME   = "Default";
    private       IThemeService _themeService;

    public ColorThemeService()
    {
        LoadSavedTheme();
    }

    public string CurrentColorTheme { get; private set; } = DEFAULT_THEME;

    public IReadOnlyList<string> AvailableColorThemes => new List<string>
    {
        DEFAULT_THEME,
        "Windows311"
    };

    public void SetThemeService(IThemeService themeService)
    {
        _themeService = themeService;

        // Reapply the current theme now that we have the theme service
        if(CurrentColorTheme != DEFAULT_THEME) ReapplyCurrentTheme();
    }

    public void ReapplyCurrentTheme()
    {
        // Force refresh of the current theme
        ApplyColorTheme(CurrentColorTheme);
    }

    public void ApplyColorTheme(string themeName)
    {
        if(!AvailableColorThemes.Contains(themeName)) return;

        CurrentColorTheme = themeName;

        // Save preference
        try
        {
            ApplicationDataContainer localSettings = ApplicationData.Current.LocalSettings;
            localSettings.Values[COLOR_THEME_KEY] = themeName;
        }
        catch
        {
            // Silently fail
        }

        // Apply the theme by rebuilding merged dictionaries
        Application app = Application.Current;

        if(app?.Resources == null) return;

        // Store the existing merged dictionaries (except color overrides)
        var existingDictionaries = app.Resources.MergedDictionaries
                                      .Where(d => d.Source?.OriginalString?.Contains("ColorPaletteOverride") != true)
                                      .ToList();

        // Clear all merged dictionaries
        app.Resources.MergedDictionaries.Clear();

        // Re-add the existing dictionaries
        foreach(ResourceDictionary dict in existingDictionaries) app.Resources.MergedDictionaries.Add(dict);

        // Add the new color theme if not default
        if(themeName != DEFAULT_THEME)
        {
            var newDictionary = new ResourceDictionary
            {
                Source = new Uri("ms-appx:///Styles/ColorPaletteOverride.xaml")
            };

            app.Resources.MergedDictionaries.Add(newDictionary);
        }

        // Force UI refresh by toggling the theme temporarily
        ForceThemeRefresh();
    }

    private void LoadSavedTheme()
    {
        try
        {
            ApplicationDataContainer localSettings = ApplicationData.Current.LocalSettings;

            if(localSettings.Values.ContainsKey(COLOR_THEME_KEY))
            {
                var savedTheme = localSettings.Values[COLOR_THEME_KEY] as string;

                if(!string.IsNullOrEmpty(savedTheme) && AvailableColorThemes.Contains(savedTheme))
                {
                    CurrentColorTheme = savedTheme;
                    ApplyColorTheme(CurrentColorTheme);
                }
            }
        }
        catch
        {
            // If loading fails, use default theme
        }
    }

    private async void ForceThemeRefresh()
    {
        if(_themeService == null) return;

        try
        {
            // Get current theme
            AppTheme currentTheme = _themeService.Theme;

            // Toggle to opposite theme briefly
            AppTheme tempTheme = currentTheme == AppTheme.Light ? AppTheme.Dark : AppTheme.Light;
            await _themeService.SetThemeAsync(tempTheme);

            // Small delay to ensure the change is applied
            await Task.Delay(50);

            // Switch back to original theme
            await _themeService.SetThemeAsync(currentTheme);
        }
        catch
        {
            // Silently fail
        }
    }
}