Starting from Windows 8 theme of WPF button by default flat. If require Vista theme, a.k.a. Chrome decorator, at Windows 8 and up theme can be apply by next approach.

BuildVistaButtonFactory.cs

/*
 *
 * The MIT License (MIT)
 *
 * Copyright (c) 2018 https://github.com/TheVice/
 * 
 * As reference used program samples
 * BuildButtonFactory from chapter 11 - "Single-Child Elements" of "Applications = Code + Markup" by Charles Petzold (2006) (ISBN 978-0-7356-1957-9)
 * and
 * ControlTemplateBrowser from chapter 15 - "Control Templates" at "Dissecting Controls" section of 
 * "Pro WPF in C# 2008 Windows Presentation Foundation with .NET 3.5" Second Edition by Matthew MacDonald (2008) (ISBN 978-1-4302-0576-0)
 *
 */

using System;
using System.IO;
using System.Xml;
using System.Text;
using System.Windows;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Markup;
using System.Windows.Controls;
using System.Windows.Documents;
using Microsoft.Windows.Themes;
using System.Windows.Controls.Primitives;

/*
 * Microsoft.Windows.Themes can be found in references at next locations - depend on Theme name and version of .NET Framework.
 * 
 * "C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\v3.0\PresentationFramework.Aero.dll"
 * "C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\v3.0\PresentationFramework.Luna.dll"
 * "C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\v3.0\PresentationFramework.Royale.dll"
 * 
 * "C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0\PresentationFramework.Aero.dll"
 * "C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0\PresentationFramework.Luna.dll"
 * "C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0\PresentationFramework.Royale.dll"
 * 
 */

namespace WpfApplication
{
    class BuildVistaButtonFactory : Window
    {
        ControlTemplate GetControlTemplateFromXmlFile(string path)
        {
            using (FileStream fileStream = new FileStream(path, FileMode.Open, FileAccess.Read))
            {
                using (XmlReader xmlReader = XmlReader.Create(fileStream))
                {
                    try
                    {
                        return (ControlTemplate)XamlReader.Load(xmlReader);
                    }
                    catch (XamlParseException)
                    {
                        string content = File.ReadAllText(path);
                        using (StringReader stringReader = new StringReader(content))
                        {
                            using (XmlReader xmlReader_ = XmlReader.Create(stringReader))
                            {
                                return (ControlTemplate)XamlReader.Load(xmlReader_);
                            }
                        }
                    }
                }
            }
        }

        string GetXamlFromControlTemplate(ControlTemplate template)
        {
            StringBuilder stringBuilder = new StringBuilder();

            XmlWriterSettings settings = new XmlWriterSettings();
            settings.Indent = true;

            using (XmlWriter writer = XmlWriter.Create(stringBuilder, settings))
            {
                XamlWriter.Save(template, writer);
            }

            return stringBuilder.ToString();
        }

        void ButtonOnClick(object sender, RoutedEventArgs args)
        {
            MessageBox.Show("You clicked the button!", Title);
        }

        public BuildVistaButtonFactory()
        {
            ControlTemplate template = new ControlTemplate(typeof(ButtonBase));

            FrameworkElementFactory factoryDecorator =
                new FrameworkElementFactory(typeof(ButtonChrome));

            factoryDecorator.Name = "Chrome";
            factoryDecorator.SetValue(Control.BackgroundProperty, new TemplateBindingExtension(Panel.BackgroundProperty));
            factoryDecorator.SetValue(Control.BorderBrushProperty, new TemplateBindingExtension(Border.BorderBrushProperty));
            factoryDecorator.SetValue(ButtonChrome.RenderDefaultedProperty, new TemplateBindingExtension(Button.IsDefaultedProperty));
            factoryDecorator.SetValue(ButtonChrome.RenderMouseOverProperty, new TemplateBindingExtension(UIElement.IsMouseOverProperty));
            factoryDecorator.SetValue(ButtonChrome.RenderPressedProperty, new TemplateBindingExtension(ButtonBase.IsPressedProperty));
            factoryDecorator.SetValue(FrameworkElement.NameProperty, "Chrome");
            factoryDecorator.SetValue(UIElement.SnapsToDevicePixelsProperty, true);

            FrameworkElementFactory factoryContent = new FrameworkElementFactory(typeof(ContentPresenter));
            factoryContent.SetValue(ContentPresenter.RecognizesAccessKeyProperty, true);
            factoryContent.SetValue(ContentControl.ContentProperty, new TemplateBindingExtension(ContentControl.ContentProperty));
            factoryContent.SetValue(ContentControl.ContentTemplateProperty, new TemplateBindingExtension(ContentControl.ContentTemplateProperty));
            factoryContent.SetValue(ContentPresenter.ContentStringFormatProperty, new TemplateBindingExtension(ContentControl.ContentStringFormatProperty));
            factoryContent.SetValue(FrameworkElement.MarginProperty, new TemplateBindingExtension(Control.PaddingProperty));
            factoryContent.SetValue(Control.HorizontalAlignmentProperty, new TemplateBindingExtension(Control.HorizontalAlignmentProperty));
            factoryContent.SetValue(Control.VerticalAlignmentProperty, new TemplateBindingExtension(Control.VerticalAlignmentProperty));
            factoryContent.SetValue(UIElement.SnapsToDevicePixelsProperty, new TemplateBindingExtension(UIElement.SnapsToDevicePixelsProperty));

            factoryDecorator.AppendChild(factoryContent);

            template.VisualTree = factoryDecorator;

            Trigger trig = new Trigger();
            trig.Property = UIElement.IsKeyboardFocusedProperty;
            trig.Value = true;

            Setter set = new Setter();
            set.Property = ButtonChrome.RenderDefaultedProperty;
            set.Value = true;
            set.TargetName = "Chrome";

            trig.Setters.Add(set);

            template.Triggers.Add(trig);

            trig = new Trigger();
            trig.Property = ToggleButton.IsCheckedProperty;
            trig.Value = true;

            set = new Setter();
            set.Property = ButtonChrome.RenderPressedProperty;
            set.Value = true;
            set.TargetName = "Chrome";

            trig.Setters.Add(set);

            template.Triggers.Add(trig);

            trig = new Trigger();
            trig.Property = UIElement.IsEnabledProperty;
            trig.Value = false;

            set = new Setter();
            set.Property = TextElement.ForegroundProperty;
            set.Value = new SolidColorBrush(Color.FromArgb(255, 173, 173, 173));

            trig.Setters.Add(set);

            template.Triggers.Add(trig);

            //string code = GetXamlFromControlTemplate(template);

            Button btn = new Button();

            btn.Template = template;

            btn.Content = "Button with Custom Template";
            btn.Padding = new Thickness(20);
            btn.FontSize = 24;
            btn.HorizontalAlignment = HorizontalAlignment.Center;
            btn.VerticalAlignment = VerticalAlignment.Center;
            btn.Click += ButtonOnClick;
            Content = btn;
        }

        [STAThread]
        public static void Main()
        {
            Application app = new Application();
            app.Run(new BuildVistaButtonFactory());
        }
    }
}