protected override void OnLaunched(LaunchActivatedEventArgs args)
{
    ResourceDictionary rootDictionary = Application.Current.Resources;

    ThemeService.InitializeThemes(new MainViewModel(), ThemeMode.SingleDictionary); // before Window.Activate
    rootDictionary["Themes"] = ThemeService.CurrentViewModel;

    _window = new MainWindow();
    _window.Activate();
}

internal class MainViewModel : ThemeViewModelBase
{
    public ThemeValue Background { get; } = new();
    public ThemeValue CornerRadiusSmall { get; } = new();

    public override void ApplyTheme(ThemeDictionary themeDictionary)
    {
        if (themeDictionary["Background"] is SolidColorBrush backgroundBrush)
        {
          Background.Value = backgroundBrush;
        }

        if (themeDictionary["CornerRadiusSmall"] is CornerRadius smallCorner)
        {
          CornerRadiusSmall.Value = smallCorner;
        }
    }
}

internal class MainViewModel : ThemeViewModelBase
{
    public SubBrushesViewModel ThemeBrushes { get; } = new();
    public SubCornersViewModel ThemeCorners { get; } = new();

    public override void ApplyBrushTheme(ThemeDictionary themeDictionary)
    {
      ThemeBrushes.ApplyTheme(themeDictionary);
    }

    public override void ApplyCornerTheme(ThemeDictionary themeDictionary)
    {
      ThemeCorners.ApplyTheme(themeDictionary);
    }
}

internal class SubBrushesViewModel : ThemeViewModelBase
{
    public ThemeValue Background { get; } = new();

    public override void ApplyTheme(ThemeDictionary themeDictionary)
    {
        if (themeDictionary["Background"] is SolidColorBrush backgroundBrush)
        {
          Background.Value = backgroundBrush;
        }
    }
}

internal class SubCornersViewModel : ThemeViewModelBase
{
    public ThemeValue CornerRadiusSmall { get; } = new();

    public override void ApplyTheme(ThemeDictionary themeDictionary)
    {
        if (themeDictionary["CornerRadiusSmall"] is CornerRadius smallCorner)
        {
          CornerRadiusSmall.Value = smallCorner;
        }
    }
}

internal class MainViewModel : ThemeViewModelBase
{
    public ThemeValue BorderSmall { get; } = new();
    public SubBrushesViewModel ThemeBrushes { get; } = new();

    public override void ApplyTheme(ThemeDictionary themeDictionary)
    {
        if (themeDictionary["BorderSmall"] is Thickness border)
        {
          BorderSmall.Value = border;
        }
    }

    public override void ApplyBrushTheme(ThemeDictionary themeDictionary)
    {
      ThemeBrushes.ApplyTheme(themeDictionary);
    }
}

internal class SubBrushesViewModel : ThemeViewModelBase
{
    public ThemeValue Background { get; } = new();

    public override void ApplyTheme(ThemeDictionary themeDictionary)
    {
        if (themeDictionary["Background"] is SolidColorBrush brush)
        {
          Background.Value = brush;
        }
    }
}

ThemeService.ApplyTheme("Light");