如果你想改变窗口的字体，你可以使用Fontfamily。
首先，你应该注册字体文件.

namespace MyMauiApp
{
    public static class MauiProgram
    {
        public static MauiApp CreateMauiApp()
        {
            var builder = MauiApp.CreateBuilder();
            builder
                .UseMauiApp<App>()
                .ConfigureFonts(fonts =>
                {
                    fonts.AddFont("Lobster-Regular.ttf", "Lobster");
                });

            return builder.Build();
        }
    }  
}

其次，可以按平台设置字体属性。OnPlatform和On类可在XAML中用于按平台设置字体属性。下面的示例设置不同的字体系列和大小：

<Label Text="Different font properties on different platforms"
       FontSize="{OnPlatform iOS=20, Android=22, WinUI=24}">
    <Label.FontFamily>
        <OnPlatform x:TypeArguments="x:String">
            <On Platform="iOS" Value="MarkerFelt-Thin" />
            <On Platform="Android" Value="Lobster-Regular" />
            <On Platform="WinUI" Value="ArimaMadurai-Black" />
        </OnPlatform>
    </Label.FontFamily>
</Label>