3.ColorView，使用Semi.Avalonia.ColorPicker，Nuget获取就行 

4.样式预览

以下是测试专用方法，动态改变默认背景色，实际上你只要在你的资源里设置以下    <SolidColorBrush x:Key="WindowDefaultBackground">#336699</SolidColorBrush>，就可以了，这个资源文件在App.axaml中引用以下就生效了

 5.WindowBackground.axaml

<UserControl 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"
			 xmlns:colorPicker="clr-namespace:Semi.Avalonia.ColorPicker;assembly=Semi.Avalonia.ColorPicker"
             xmlns:controls="using:Avalonia.Controls"
             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
             x:Class="AvaloniaDemo.Pages.WindowBackground">
	<StackPanel Spacing="20">
		<StackPanel
			 VerticalAlignment="Top"
			 Orientation="Horizontal"
			 Spacing="20">
			<ColorView ColorSpectrumShape="Ring" x:Name="ColorV" Loaded="ColorVLoaded"/>
		</StackPanel>
	</StackPanel>
</UserControl>

6.WindowBackground.axaml.cs 

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

namespace AvaloniaDemo.Pages;

public partial class WindowBackground : UserControl
{
    public EventHandler<ColorChangedEventArgs>? ColorChangedHandler;
    public WindowBackground()
    {
        InitializeComponent();
    }

    private void ColorVLoaded(object? sender, RoutedEventArgs e)
    {
        if (sender is ColorView colorView)
            colorView.ColorChanged += ColorChanged;
    }

    private void ColorChanged(object? sender, ColorChangedEventArgs e)
    {
        ColorChangedHandler?.Invoke(this, e);
    }
}

7.MainWindow.axaml.cs

using Avalonia.Controls;
using Avalonia.Markup.Xaml;
using Avalonia.Media;
using Avalonia.Themes.Fluent;
using AvaloniaDemo.Pages;

namespace AvaloniaDemo.Views
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            this.InitializeComponent();
        }

        private void TabLoaded(object? sender, Avalonia.Interactivity.RoutedEventArgs e)
        {
            if (sender is WindowBackground windowBackground)
                windowBackground.ColorChangedHandler += ColorChanged;
        }

        private void ColorChanged(object? sender, ColorChangedEventArgs e)
        {
            Resources["WindowDefaultBackground"] = new SolidColorBrush(e.NewColor);
        }

        private void InitializeComponent()
        {
            AvaloniaXamlLoader.Load(this);

        }
    }
}

8. 标题栏颜色问题

参考 https://github.com/AvaloniaUI/Avalonia/issues/186

解决方案是，设置 SystemDecorations="None"，就是不要标题栏了，自己画一个：类似这个样子

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="using:AvaloniaDemo.ViewModels"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:pages="using:AvaloniaDemo.Pages"
		mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="AvaloniaDemo.Views.MainWindow"
        x:DataType="vm:MainWindowViewModel"
        Icon="/Assets/avalonia-logo.ico"
		SystemDecorations="None"
        Title="AvaloniaDemo">

	<Design.DataContext>
		<!-- This only sets the DataContext for the previewer in an IDE,
             to set the actual DataContext for runtime, set the DataContext property in code (look at App.axaml.cs) -->
		<vm:MainWindowViewModel/>
	</Design.DataContext>
	<Grid RowDefinitions="40,*">
		<StackPanel Background="Pink" Orientation="Horizontal" VerticalAlignment="Center">
			<TextBlock Text="画一个标题栏"/>
			<Button Content="×" FontSize="25" Margin="1010 0 0 0"/>
		</StackPanel>
		<TabControl 
			   Name="tab"
			   Grid.Row="1"
			   Margin="8"
			   Padding="20,0,0,0"
			   HorizontalAlignment="Stretch"
			   TabStripPlacement="Left">
			<TabItem Header="DataGrid">
				<pages:DataGrid />
			</TabItem>
		
		</TabControl>
	</Grid>
</Window>