Semi.Avalonia/demo/Semi.Avalonia.Demo/Views/MainView.axaml.cs

using Avalonia;
using Avalonia.Controls;
using Avalonia.Interactivity;
using Avalonia.Styling;

namespace Semi.Avalonia.Demo.Views;

public partial class MainView : UserControl
{
    public MainView()
    {
        InitializeComponent();
    }

    private void ToggleButton_OnIsCheckedChanged(object sender, RoutedEventArgs e)
    {
        var app = Application.Current;
        if (app is not null)
        {
            var theme = app.ActualThemeVariant;
            app.RequestedThemeVariant = theme == ThemeVariant.Dark ? ThemeVariant.Default : ThemeVariant.Dark;
        }
    }
}