<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        Height="300" Width="300"
        SystemDecorations="None"
        TransparencyLevelHint="AcrylicBlur,Transparent"
        Background="Transparent"
        x:Class="AvaloniaUI.TransparentBackground"
        Title="TransparentBackground"
        PointerPressed="Window_PointerPressed">
    
    <Window.Background>
        <ImageBrush Source="avares://AvaloniaUI/Resources/Images/shapes.png"
                    Stretch="UniformToFill"
                    Opacity="0.7"/>
    </Window.Background>

    <Grid RowDefinitions="*,*,*,*">
        <Button Margin="20">A Sample Button</Button>
        <Button Margin="20"
                Grid.Row="2"
                Click="cmdClose_Click">
            Close
        </Button>
    </Grid>
</Window>

using Avalonia;
using Avalonia.Controls;
using Avalonia.Input;
using Avalonia.Interactivity;
using Avalonia.Markup.Xaml;

namespace AvaloniaUI;

public partial class TransparentBackground : Window
{
    public TransparentBackground()
    {
        InitializeComponent();
    }

    private void Window_PointerPressed(object? sender, PointerPressedEventArgs e)
    {
        if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed)
            BeginMoveDrag(e);
    }

    private void cmdClose_Click(object? sender, RoutedEventArgs e)
    {
        Close();
    }
}