WPF FindResource,Resource[key] SystemColors TryFindResource

//xaml
<Window x:Class="WpfApp3.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        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:local="clr-namespace:WpfApp3"
        mc:Ignorable="d"
        Background="{DynamicResource bgBrush}"
        WindowState="Maximized"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <Image x:Shared="True" x:Key="noSharedRS" Height="300"
               Source="Images/1.jpg"/>
    </Window.Resources>
    <StackPanel>
        <StaticResource ResourceKey="noSharedRS"/>
        <Button x:Name="btn" Height="100"/>
        <Button x:Name="btn2" Height="100"/>
        <Button x:Name="btn3" Height="100"/>
        <Button Background="{DynamicResource {x:Static SystemColors.WindowBrushKey}}" Height="100"/>
    </StackPanel>
</Window>



//cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApp3
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            this.Resources.Add("bgBrush", new SolidColorBrush(Colors.Yellow));
            this.Resources.Add("borderBrush",new SolidColorBrush(Colors.Red));
            this.Resources.Add("btnBgBrush", new SolidColorBrush(Colors.Blue)); 
            btn.Background =(Brush)this.TryFindResource("btnBgBrush");
            btn.BorderBrush = (Brush)this.TryFindResource("borderBrush");

            btn2.SetResourceReference(Button.BackgroundProperty, "borderBrush");
            btn2.SetResourceReference(Button.BorderBrushProperty, "btnBgBrush");


            btn3.Background = (Brush)this.Resources["btnBgBrush"];
            btn3.BorderBrush = (Brush)this.Resources["borderBrush"];
        }
    }
}