为了能够完全自定义这样的按钮，最好实现自己的Button控件，而不是使用CSS：

XAML语言

<?xml version="1.0" encoding="UTF-8"?>
<ContentView xmlns="http://xamarin.com/schemas/2014/forms" 
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="YourApp.Views.SomeRoundImageButton">

  <Frame
    CornerRadius="8">
    <Grid
      RowDefinitions="2*,*">
      <Image
        Grid.Row="0"
        Source="{Binding Icon}"/>
      <Label
        Grid.Row="1"
        TextColor="Blue"
        Text="{Binding Title}"/>      
    </Grid>
    <Frame.GestureRecognizers>
      <TapGestureRecognizer
        Tapped="OnButtonTapped"/>
    </Frame.GestureRecognizers>
  </Frame>

</ContentView>

代码隐藏

using System;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace YourApp.Views {
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class SomeRoundImageButton : ContentView {

        public string Title {
            get => (string)GetValue(TitleProperty);
            set => SetValue(TitleProperty, value);
        }

        public ImageSource Icon {
            get => (ImageSource)GetValue(IconProperty);
            set => SetValue(IconProperty, value);
        }

        public event EventHandler<EventArgs> ButtonTapped;

        public static readonly BindableProperty TitleProperty = BindableProperty.Create(nameof(Title), typeof(string), typeof(SomeRoundImageButton));
        public static readonly BindableProperty IconProperty = BindableProperty.Create(nameof(Icon), typeof(ImageSource), typeof(SomeRoundImageButton));

        public SomeRoundImageButton() {
            InitializeComponent();
            BindingContext = this;
        }

        private void OnButtonTapped(object sender, EventArgs e) {
            ButtonTapped?.Invoke(this, e);
        }
    }
}

用法

<ContentView.Content>
    <StackLayout>
        <views:SomeRoundImageButton
          Icon="{Binding Icon}"
          Title="{Binding Title}"
          WidthRequest="80"
          HeightRequest="80"
          ButtonTapped="Clicked"/>
    </StackLayout>
</ContentView.Content>

这给了你更多的设计自由，你可以让图像更大或更小，添加更多行的文本到标签，定义CornerRadius的大小，设置各种其他属性...
显然，您需要尝试一下它来找出预期的设计，但我希望这会有所帮助。注意，这只是一种方法。在Xamarin.Forms（和.NET MAUI）中，有许多方法可以实现相同的功能。