Add UseDarkMode property to DevToolsOptions

src/Avalonia.Diagnostics/Diagnostics/DevToolsOptions.cs

@@ -41,5 +41,10 @@ namespace Avalonia.Diagnostics
         /// <remarks>Default handler is <see cref="Screenshots.FilePickerHandler"/></remarks>
         public IScreenshotHandler ScreenshotHandler { get; set; }
           = Convetions.DefaultScreenshotHandler;
+
+        /// <summary>
+        /// Gets or sets whether DevTools should use the dark mode theme
+        /// </summary>
+        public bool UseDarkMode { get; set; }
     }
 }

src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml.cs

@@ -11,6 +11,7 @@ using Avalonia.Input;
 using Avalonia.Input.Raw;
 using Avalonia.Markup.Xaml;
 using Avalonia.Styling;
+using Avalonia.Themes.Default;
 using Avalonia.VisualTree;
 
 namespace Avalonia.Diagnostics.Views
@@ -242,7 +243,17 @@ namespace Avalonia.Diagnostics.Views
 
         private void RootClosed(object? sender, EventArgs e) => Close();
 
-        public void SetOptions(DevToolsOptions options) =>
+        public void SetOptions(DevToolsOptions options)
+        {
             (DataContext as MainViewModel)?.SetOptions(options);
+
+            if (options.UseDarkMode)
+            {
+                if (Styles[0] is SimpleTheme st)
+                {
+                    st.Mode = SimpleThemeMode.Dark;
+                }                
+            }
+        }
     }
 }