StyleManager helper class, allowing to change Avalonia theme which a Window uses dynamically at runtime. See: https://medium.com/swlh/cross-platform-gui-for-dotnet-applications-bbd284709600

StyleManager.cs

public sealed class StyleManager
{
    public enum Theme { Citrus, Sea, Rust, Candy, Magma }

    private readonly StyleInclude _magmaStyle = CreateStyle("avares://Citrus.Avalonia/Magma.xaml");
    private readonly StyleInclude _candyStyle = CreateStyle("avares://Citrus.Avalonia/Candy.xaml");
    private readonly StyleInclude _citrusStyle = CreateStyle("avares://Citrus.Avalonia/Citrus.xaml");
    private readonly StyleInclude _rustStyle = CreateStyle("avares://Citrus.Avalonia/Rust.xaml");
    private readonly StyleInclude _seaStyle = CreateStyle("avares://Citrus.Avalonia/Sea.xaml");
    private readonly Window _window;

    public StyleManager(Window window)
    {
        _window = window;

        // We add the style to the window styles section, so it
        // will override the default style defined in App.xaml. 
        window.Styles.Add(_citrusStyle);
    }

    public Theme CurrentTheme { get; private set; } = Theme.Citrus;

    public void UseTheme(Theme theme)
    {
        // Here, we change the first style in the main window styles
        // section, and the main window instantly refreshes. Remember
        // to invoke such methods from the UI thread.
        _window.Styles[0] = theme switch
        {
            Theme.Citrus => _citrusStyle,
            Theme.Sea => _seaStyle,
            Theme.Rust => _rustStyle,
            Theme.Candy => _candyStyle,
            Theme.Magma => _magmaStyle,
            _ => throw new ArgumentOutOfRangeException(nameof(theme))
        };

        CurrentTheme = theme;
    }

    private static StyleInclude CreateStyle(string url)
    {
        var self = new Uri("resm:Styles?assembly=Citrus.Avalonia.Sandbox");
        return new StyleInclude(self)
        {
            Source = new Uri(url)
        };
    }
}