Dark Mode issues Fixes

src/System.Windows.Forms.Primitives/src/NativeMethods.txt

@@ -221,6 +221,7 @@ GetUpdateRgn
 GetUserObjectInformation
 GetViewportExtEx
 GetWindow
+GetWindowTheme
 GetWindowDpiAwarenessContext
 GetWindowPlacement
 GetWorldTransform

src/System.Windows.Forms/System/Windows/Forms/Application.cs

@@ -313,6 +313,7 @@ public static void SetColorMode(SystemColorMode systemColorMode)
             {
                 SystemColors.UseAlternativeColorSet = darkModeEnabled;
                 NotifySystemEventsOfColorChange();
+                PInvokeCore.EnumWindows(SendThemeChanged);
             }
         }
 

src/System.Windows.Forms/System/Windows/Forms/Control.cs

@@ -10306,15 +10306,16 @@ protected virtual void SetVisibleCore(bool value)
                 {
                     // We shouldn't mess with the color mode if users haven't specifically set it.
                     // https://github.com/dotnet/winforms/issues/12014
-                    if (value && Application.ColorModeSet)
+                    // I Don't think we should be doing this for Forms here , but it is the same as the old code but execute Form Control.
+                    if (value && Application.ColorModeSet && this is not Form)
                     {
                         PrepareDarkMode(HWND, Application.IsDarkModeEnabled);
                     }
 
                     PInvoke.ShowWindow(HWND, value ? ShowParams : SHOW_WINDOW_CMD.SW_HIDE);
                 }
             }
-#pragma warning restore WFO5001
+#pragma warning restore WFO5001w
             else if (IsHandleCreated || (value && _parent?.Created == true))
             {
                 // We want to mark the control as visible so that CreateControl
@@ -12481,6 +12482,24 @@ protected virtual void WndProc(ref Message m)
             case PInvokeCore.WM_SETTINGCHANGE:
                 if (GetExtendedState(ExtendedStates.InterestedInUserPreferenceChanged) && GetTopLevel())
                 {
+                    // need Handle immediately drawing of Scrollbars.
+#pragma warning disable WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
+                    Application.SetColorMode(Application.ColorMode);
+                    const string DarkModeThemeIdentifier = $"{DarkModeIdentifier}_{ExplorerThemeIdentifier}";
+                    PInvoke.SetWindowTheme(m.HWND,
+                        Application.IsDarkModeEnabled
+                        ? DarkModeThemeIdentifier
+                        : default,
+                        null);
+                    PInvoke.RedrawWindow(
+                        m.HWND,
+                        lprcUpdate: (RECT*)null,
+                        HRGN.Null,
+                        REDRAW_WINDOW_FLAGS.RDW_INVALIDATE
+                        | REDRAW_WINDOW_FLAGS.RDW_FRAME
+                        | REDRAW_WINDOW_FLAGS.RDW_ERASE
+                        | REDRAW_WINDOW_FLAGS.RDW_ALLCHILDREN);
+#pragma warning restore WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
                     SYSTEM_PARAMETERS_INFO_ACTION action = (SYSTEM_PARAMETERS_INFO_ACTION)(uint)m.WParamInternal;
 
                     // Left here for debugging purposes.

src/System.Windows.Forms/System/Windows/Forms/Controls/TreeView/TreeView.cs

@@ -3398,6 +3398,16 @@ protected override unsafe void WndProc(ref Message m)
             case PInvokeCore.WM_SYSCOLORCHANGE:
                 PInvokeCore.SendMessage(this, PInvoke.TVM_SETINDENT, (WPARAM)Indent);
                 base.WndProc(ref m);
+                break;
+            case PInvokeCore.WM_THEMECHANGED:
+#pragma warning disable WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
+                if (Application.IsDarkModeEnabled)
+                {
+                    HTHEME theme = PInvoke.GetWindowTheme(m.HWND);
+                    PInvoke.CloseThemeData(theme);
+                    m.ResultInternal = (LRESULT)0;
+                }
+
                 break;
             case PInvokeCore.WM_SETFOCUS:
                 // If we get focus through the LButtonDown .. we might have done the validation...

src/System.Windows.Forms/System/Windows/Forms/Form.cs

@@ -2265,6 +2265,9 @@ private unsafe void SetFormCornerPreferenceInternal(FormCornerPreference cornerP
     ///   those changes, as the Win32 API does not provide a mechanism to retrieve the current title
     ///   bar color.
     ///  </para>
+    /// <para>
+    ///   Note: Setting <see cref="FormBorderColor"/> to <see cref="Color.Transparent"/> suppresses the drawing of the window border, allowing the form to have rounded corners without a visible border. Setting <see cref="FormBorderColor"/> to <see cref="Color.Empty"/> resets it to the system default color.
+    /// </para>
     ///  <para>
     ///   The property only reflects the value that was previously set using this property. The
     ///   <see cref="FormBorderColorChanged"/> event is raised accordingly when the value is
@@ -2327,6 +2330,9 @@ protected virtual void OnFormBorderColorChanged(EventArgs e)
     ///   those changes, as the Win32 API does not provide a mechanism to retrieve the current title
     ///   bar color.
     ///  </para>
+    /// <para>
+    ///   Note: Setting <see cref="FormBorderColor"/> to <see cref="Color.Empty"/> resets it to the system default color.
+    /// </para>
     ///  <para>
     ///   The property only reflects the value that was previously set using this property. The
     ///   <see cref="FormCaptionBackColorChanged"/> event is raised accordingly when the value is
@@ -2395,6 +2401,9 @@ protected virtual void OnFormCaptionBackColorChanged(EventArgs e)
     ///   <see cref="FormCaptionTextColorChanged"/> event is raised accordingly when the value is
     ///   changed, which allows the property to be participating in binding scenarios.
     ///  </para>
+    /// <para>
+    ///   Note: Setting <see cref="FormBorderColor"/> to <see cref="Color.Empty"/> resets it to the system default color.
+    /// </para>
     /// </remarks>
     [SRCategory(nameof(SR.CatWindowStyle))]
     [SRDescription(nameof(SR.FormCaptionTextColorDescr))]
@@ -2453,7 +2462,11 @@ private unsafe Color GetFormAttributeColorInternal(DWMWINDOWATTRIBUTE dmwWindowA
 
     private unsafe void SetFormAttributeColorInternal(DWMWINDOWATTRIBUTE dmwWindowAttribute, Color color)
     {
-        COLORREF colorRef = color;
+        COLORREF colorRef = color.IsEmpty ? (COLORREF)(Color.White.ToArgb()) : (COLORREF)color;
+        if (color == Color.Transparent && dmwWindowAttribute == DWMWINDOWATTRIBUTE.DWMWA_BORDER_COLOR)
+        {
+            colorRef = (COLORREF)(Color.FromArgb(0xFF, 0xFF, 0xFF, 0xFE).ToArgb());
+        }
 
         PInvoke.DwmSetWindowAttribute(
             HWND,
@@ -4509,6 +4522,28 @@ public event DpiChangedEventHandler? DpiChanged
         remove => Events.RemoveHandler(s_dpiChangedEvent, value);
     }
 
+    /// <summary>
+    /// Handles the WM_THEMECHANGED message for the form. Updates the window's immersive dark mode attribute
+    /// if the application's color mode is set, ensuring the form reflects the current system theme (light or dark).
+    ///</summary>
+    ///<param name="m"> The Windows message containing theme change information.</param>
+    private unsafe void WmThemeChanged(ref Message m)
+    {
+        base.WndProc(ref m);
+        if (Application.ColorModeSet && GetTopLevel())
+        {
+#pragma warning disable WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
+            BOOL value = Application.IsDarkModeEnabled;
+#pragma warning restore WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
+
+            PInvoke.DwmSetWindowAttribute(
+                m.HWND,
+                DWMWINDOWATTRIBUTE.DWMWA_USE_IMMERSIVE_DARK_MODE,
+                &value,
+                (uint)sizeof(BOOL)).AssertSuccess();
+        }
+    }
+
     /// <summary>
     ///  Handles the WM_DPICHANGED message
     /// </summary>
@@ -6535,11 +6570,34 @@ private void WmExitSizeMove()
     /// <summary>
     ///  WM_CREATE handler
     /// </summary>
-    private void WmCreate(ref Message m)
+    private unsafe void WmCreate(ref Message m)
     {
         base.WndProc(ref m);
         PInvoke.GetStartupInfo(out STARTUPINFOW si);
+#pragma warning disable WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
+        if (TopLevel && IsHandleCreated)
+        {
+            FormCornerPreference formCornerPreference = Properties.GetValueOrDefault(s_propFormCornerPreference, FormCornerPreference.Default);
+            SetFormCornerPreferenceInternal(formCornerPreference);
 
+            Color colorValue = Properties.GetValueOrDefault(s_propFormCaptionTextColor, Color.Empty);
+            SetFormAttributeColorInternal(DWMWINDOWATTRIBUTE.DWMWA_TEXT_COLOR, colorValue);
+
+            colorValue = Properties.GetValueOrDefault(s_propFormCaptionBackColor, Color.Empty);
+            SetFormAttributeColorInternal(DWMWINDOWATTRIBUTE.DWMWA_CAPTION_COLOR, colorValue);
+
+            colorValue = Properties.GetValueOrDefault(s_propFormBorderColor, Color.Empty);
+            SetFormAttributeColorInternal(DWMWINDOWATTRIBUTE.DWMWA_BORDER_COLOR, colorValue);
+        }
+
+        // Set the theme for the form. This is needed to set the dark mode theme
+        if (Application.IsDarkModeEnabled && Application.ColorModeSet)
+        {
+            BOOL value = true;
+            PInvoke.DwmSetWindowAttribute(HWND, DWMWINDOWATTRIBUTE.DWMWA_USE_IMMERSIVE_DARK_MODE, &value, sizeof(int));
+        }
+
+#pragma warning restore WFO5001 // Type is for evaluation purposes only and is subject to chang
         // If we've been created from explorer, it may
         // force us to show up normal. Force our current window state to
         // the specified state, unless it's _specified_ max or min
@@ -7116,6 +7174,9 @@ protected override void WndProc(ref Message m)
             case PInvokeCore.WM_DPICHANGED:
                 WmDpiChanged(ref m);
                 break;
+            case PInvokeCore.WM_THEMECHANGED:
+                WmThemeChanged(ref m);
+                break;
             default:
                 base.WndProc(ref m);
                 break;