gregExtractor/GUI/App.axaml.cs

using Avalonia;
using Avalonia.Controls.ApplicationLifetimes;
using Avalonia.Markup.Xaml;
using gregExtractor.GUI.ViewModels;

namespace gregExtractor.GUI;

public partial class App : Application
{
    public override void Initialize()
    {
        AvaloniaXamlLoader.Load(this);
        RequestedThemeVariant = Avalonia.Styling.ThemeVariant.Dark;
    }

    public override void OnFrameworkInitializationCompleted()
    {
        if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
        {
            desktop.MainWindow = new MainWindow
            {
                DataContext = new MainViewModel(),
            };
        }

        base.OnFrameworkInitializationCompleted();
    }
}