diff --git a/src/EasyWindowsApplication/Core/Application.cs b/src/EasyWindowsApplication/Core/Application.cs
index de0e00b..c1b4079 100644
--- a/src/EasyWindowsApplication/Core/Application.cs
+++ b/src/EasyWindowsApplication/Core/Application.cs
@@ -1,4 +1,5 @@
-using EasyWindowsApplication.Common;
+using System.Diagnostics;
+using EasyWindowsApplication.Common;
 using EasyWindowsApplication.Share;
 using EasyWindowsApplication.Core.LayoutEngine;
 using EasyWindowsApplication.Core.Windowing;
@@ -64,6 +65,11 @@ public void Initialize()
         ControlActivatorRegistry.EnsureInitialized();
         _router = new MasterRouter(_registry);
 
+        Debug.Assert(
+            UiDefaultsProvider.IsInitialized,
+            "UiDefaultsProvider debe inicializarse (UseWinApi / UseWpf / …) " +
+            "antes de registrar la clase de ventana.");
+
         foreach (var window in _windows)
         {
             if (window.IsAlternative)
diff --git a/src/EasyWindowsApplication/Core/MasterRouter.cs b/src/EasyWindowsApplication/Core/MasterRouter.cs
index 7f162f3..8b1c78c 100644
--- a/src/EasyWindowsApplication/Core/MasterRouter.cs
+++ b/src/EasyWindowsApplication/Core/MasterRouter.cs
@@ -265,10 +265,13 @@ internal nint WndProc(nint hwnd, uint msg, nint wParam, nint lParam)
         // Handle WM_ERASEBKGND — paint window background, then LayoutGroup backgrounds on top
         if (msg == WM.ERASEBKGND)
         {
+            bool painted = false;
+
             if (_windowBrushes.TryGetValue(hwnd, out var windowBrush))
             {
                 Win32.GetClientRect(hwnd, out RECT rect);
                 Win32.FillRect(wParam, ref rect, windowBrush);
+                painted = true;
             }
 
             if (_layoutGroupBackgrounds.TryGetValue(hwnd, out var backgrounds))
@@ -288,9 +291,10 @@ internal nint WndProc(nint hwnd, uint msg, nint wParam, nint lParam)
                     Win32.FillRect(wParam, ref r, brush);
                     Win32.DeleteObject(brush);
                 }
+                painted = true;
             }
 
-            return 1;
+            if (painted) return 1;
         }
 
         // Handle WM_CTLCOLOR* — return control background brush
diff --git a/src/EasyWindowsApplication/Core/UiDefaults/CoreUiDefaults.cs b/src/EasyWindowsApplication/Core/UiDefaults/CoreUiDefaults.cs
index ac02d89..550feae 100644
--- a/src/EasyWindowsApplication/Core/UiDefaults/CoreUiDefaults.cs
+++ b/src/EasyWindowsApplication/Core/UiDefaults/CoreUiDefaults.cs
@@ -23,10 +23,10 @@ internal abstract class CoreUiDefaults : IDefaultUiValues
     public abstract FontSpec DefaultFont { get; }
 
     public virtual Color DefaultForeground
-        => ColorFromSysColor(Win32.GetSysColor(COLOR_WINDOWTEXT));
+        => ColorFromSysColor(Win32.GetSysColor(Win32.COLOR_WINDOWTEXT));
 
     public virtual Color DefaultBackground
-        => ColorFromSysColor(Win32.GetSysColor(COLOR_WINDOW));
+        => ColorFromSysColor(Win32.GetSysColor(Win32.COLOR_WINDOW));
 
     public virtual Thickness DefaultPadding => new(3);
     public virtual int DefaultControlSpacing => 6;
@@ -43,10 +43,4 @@ protected static Color ColorFromSysColor(int colorRef)
             (byte)(colorRef & 0xFF),
             (byte)((colorRef >> 8) & 0xFF),
             (byte)((colorRef >> 16) & 0xFF));
-
-    // SysColor indices (winuser.h)
-    private const int COLOR_WINDOW = 5;
-    private const int COLOR_WINDOWTEXT = 8;
-    private const int COLOR_BTNFACE = 15;
-    private const int COLOR_BTNTEXT = 18;
 }
diff --git a/src/EasyWindowsApplication/Core/UiDefaults/UiDefaultsProvider.cs b/src/EasyWindowsApplication/Core/UiDefaults/UiDefaultsProvider.cs
index c8fac77..ab139a8 100644
--- a/src/EasyWindowsApplication/Core/UiDefaults/UiDefaultsProvider.cs
+++ b/src/EasyWindowsApplication/Core/UiDefaults/UiDefaultsProvider.cs
@@ -10,6 +10,7 @@ private sealed class CoreFallbackDefaults : CoreUiDefaults
     private static IDefaultUiValues _current = new CoreFallbackDefaults();
 
     public static IDefaultUiValues Current => _current;
+    public static bool IsInitialized => _current is not CoreFallbackDefaults;
 
     public static void Set(IDefaultUiValues defaults)
         => _current = defaults ?? throw new ArgumentNullException(nameof(defaults));
diff --git a/src/EasyWindowsApplication/Core/Win32.cs b/src/EasyWindowsApplication/Core/Win32.cs
index 7612392..15147cd 100644
--- a/src/EasyWindowsApplication/Core/Win32.cs
+++ b/src/EasyWindowsApplication/Core/Win32.cs
@@ -4,6 +4,12 @@ namespace EasyWindowsApplication.Core;
 
 internal static partial class Win32
 {
+    // ── System Color Indices (winuser.h) ──
+    internal const int COLOR_WINDOW = 5;
+    internal const int COLOR_WINDOWTEXT = 8;
+    internal const int COLOR_BTNFACE = 15;
+    internal const int COLOR_BTNTEXT = 18;
+
     // ── Message Loop ──
     [LibraryImport("user32.dll", SetLastError = true)]
     internal static partial int GetMessageW(out MSG lpMsg, nint hWnd, uint wMsgFilterMin, uint wMsgFilterMax);
diff --git a/src/EasyWindowsApplication/Core/Windowing/Procedures.cs b/src/EasyWindowsApplication/Core/Windowing/Procedures.cs
index 63c4d1c..64ee4bc 100644
--- a/src/EasyWindowsApplication/Core/Windowing/Procedures.cs
+++ b/src/EasyWindowsApplication/Core/Windowing/Procedures.cs
@@ -65,7 +65,7 @@ internal static unsafe nint CreateMainWindow(
             hInstance = hInstance,
             hIcon = hIconLarge,
             hCursor = 0,
-            hbrBackground = 0,
+            hbrBackground = (nint)(Win32.COLOR_WINDOW + 1),
             lpszMenuName = 0,
             lpszClassName = classNamePtr,
             hIconSm = hIconSmall
@@ -124,7 +124,7 @@ internal static unsafe nint CreateAlternativeWindow(
             hInstance = hInstance,
             hIcon = 0,
             hCursor = 0,
-            hbrBackground = 0,
+            hbrBackground = (nint)(Win32.COLOR_WINDOW + 1),
             lpszMenuName = 0,
             lpszClassName = classNamePtr,
             hIconSm = 0
diff --git a/src/EasyWindowsApplication/Core/Windowing/Win32.cs b/src/EasyWindowsApplication/Core/Windowing/Win32.cs
index 142dc89..e8e49a3 100644
--- a/src/EasyWindowsApplication/Core/Windowing/Win32.cs
+++ b/src/EasyWindowsApplication/Core/Windowing/Win32.cs
@@ -5,6 +5,9 @@ namespace EasyWindowsApplication.Core.Windowing;
 
 internal static partial class Win32
 {
+    // ── System Color Indices (winuser.h) ──
+    internal const int COLOR_WINDOW = 5;
+
     [LibraryImport("user32.dll", SetLastError = true, StringMarshalling = StringMarshalling.Utf16)]
     internal static partial ushort RegisterClassExW(ref WNDCLASSEXW lpWndClass);