food-market/src/food-market.pos.hwcheck/App.cs

using Avalonia;
using Avalonia.Controls.ApplicationLifetimes;
using Avalonia.Styling;
using Avalonia.Themes.Fluent;

namespace foodmarket.Pos.HwCheck;

public class App : Application
{
    public override void Initialize()
    {
        Styles.Add(new FluentTheme());
        RequestedThemeVariant = ThemeVariant.Light; // светлый, читаемо (требование)
    }

    public override void OnFrameworkInitializationCompleted()
    {
        if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
            desktop.MainWindow = new MainWindow(Program.AllowTestPrint);
        base.OnFrameworkInitializationCompleted();
    }
}